GC.KeepAlive for UnmanagedImages about aforge HOT 2 OPEN

One thing that is noteworthy is that using System.Drawing.Bitmap instances with 
AForge is safe, because the UnlockBits method (which is always called after 
processing) keeps a reference to the bitmap.

Another is that dully disposing of all UnmanagedImage instances is also safe.


I'll be providing a first patch that goes through all the Filter base classes 
and fixes it for subclasses, if you're interested. But it's only just 
scratching the surface of the problem.

Attached a patch that fixes this for the areas of AForge I'm using: mostly 
filters, blob detection and color quantization.