implement adjustable seams (base loop) about loopidity HOT 1 OPEN

RecordBuffer layout

    to allow for dynamic adjustment of seams and compensation for SDL key event delay
      the following are the buffer offsets used:

        invariant                  -->
            BeginFrameN <= beginFrameN <= currentFrameN < endFrameN <= EndFrameN
        initially                  -->
            beginFrameN     = currentFrameN = BeginFrameN = BufferMarginSize
            endFrameN       = EndFrameN     = (RecordBufferSize - BufferSize)
        on initial trigger         -->
            beginFrameN     = currentFrameN - TriggerLatencySize
        on accepting trigger       -->
            if (currentFrameN > beginFrameN + MINIMUM_LOOP_DURATION) // (issue #12)
                endFrameN     = currentFrameN
                nFrames       = endFrameN - TriggerLatencySize - beginFrameN
        after each rollover        -->
            beginFrameN     = BeginFrameN
            endFrameN       = BeginFrameN + nFrames
        after first rollover       -->
            currentFrameN   = BeginFrameN + TriggerLatencySize
        after subsequent rollovers -->
            currentFrameN   = BeginFrameN

    all of the above offsets are multiples of BufferSize (aka nFramesPerPeriod)
      excepting beginFrameN and nFrames

    on each rollover (TODO: this is currently only done when copying base loops)
        set currentFrameN = BeginFrameN (+ TriggerLatencySize if base loop)
        copy tail end of RecordBuffer back to the beginning of RecordBuffer
        this will be the LeadIn of the next loop (may or may not be part of a previous loop)
            let begin = (beginFrameN + nFrames) - BufferMarginSize
        initial base loop -->
            RecordBuffer[begin] upto RecordBuffer[endFrameN]
                --> RecordBuffer[0]
        subsequent loops  -->
            RecordBuffer[begin] upto RecordBuffer[endFrameN + TriggerLatencySize]
                --> RecordBuffer[0]

    on creation of each loop
        copy the entire buffer to include per loop LeadIn
            RecordBuffer[0] upto RecordBuffer[endFrameN]
                --> NewLoop[0]

    on creation of base loops
        set beginFrameN   = BeginFrameN
        set currentFrameN = BeginFrameN + TriggerLatencySize
        set endFrameN     = BeginFrameN + nFrames
        all subsequent loops will use beginFrameN and endFrameN (dynamically) as seams

    on next rollover following creation of each loop
        copy the leading BeginFrameN + BufferMarginSize to the end of the previous loop
          regardless of creating a new loop from the current data
        this will be the leadOut of the previous loop
            RecordBuffer[BeginFrameN] upto RecordBuffer[BeginFrameN + BufferMarginSize]
                --> PrevLoop[BeginFrameN + nFrames]


                            INITIAL RECORD BUFFER
          currentFrameN                                                 // dynamic offset
           beginFrameN                                        endFrameN // dynamic offsets
           BeginFrameN                                        EndFrameN // static offsets
                |                                                 |
 |<-MarginSize->|<--------(RecordBufferSize - MarginSize)-------->|     // sizes
 |<---LeadIn--->|<-----------------RolloverRange----------------->|     // 'zones'
 |<-------------------------RecordBuffer------------------------->|     // buffer


                              INITIAL BASE LOOP
                 beginFrameN                       endFrameN                  // offsets
thisLeadInBegin       |                                |                      // pointer
       |              |                                |
 |<-?->|<-MarginSize->|<-----------nFrames------------>|<-MarginSize->|<-?->| // sizes
 |     |<---LeadIn--->|<--------RolloverRange--------->|<--LeadOut--->|     | // 'zones'
 |     |<-------------------Source-------------------->|              |     | // data
 |     |<--------------------Dest--------------------->|<--Pending--->|     | // data
 |     |<--------------------------NewLoop--------------------------->|     | // dest
 |<------------------------------RecordBuffer------------------------------>| // source


                               SUBSEQUENT LOOPS
           beginFrameN                       endFrameN                        // offsets
thisLeadInBegin |                                |                            // pointer
 |              |                                |
 |<-MarginSize->|<-----------nFrames------------>|<-MarginSize->|<----?---->| // sizes
 |<---LeadIn--->|<--------RolloverRange--------->|<--LeadOut--->|           | // 'zones'
 |<-------------------Source-------------------->|              |           | // data
 |<--------------------Dest--------------------->|<--Pending--->|           | // data
 |<--------------------------NewLoop--------------------------->|           | // dest
 |<------------------------------RecordBuffer------------------------------>| // source


                             SHIFT NEXT LEAD-IN
           beginFrameN                      endFrameN                         // offsets
                |         nextLeadInBegin       |                             // pointer
                |                |              |
 |<-MarginSize->|                |<-MarginSize->|                           | // sizes
 |<----Dest---->|                |<---Source--->|                           | // data
 |<------------------------------RecordBuffer------------------------------>| // source/dest


                        COPY PREVIOUS LEAD_OUT (TODO)
           beginFrameN                       endFrameN                        // offsets
                |           mixBegin             |                            // pointer
                |              |                 |
 |<-MarginSize->|<-MarginSize->|                 |<-MarginSize->|           | // sizes
 |              |<---Source--->|                 |<----Dest---->|           | // data
 |<---LeadIn--->|<--------RolloverRange--------->|<--LeadOut--->|           | // 'zones'
 |<--------------------------NewLoop--------------------------->|           | // dest
 |<------------------------------RecordBuffer------------------------------>| // source