Add support for gain maps about pex-renderer HOT 2 OPEN

Summary:

• a pex-loaders feature, without any dependency other than a pex-context to recover the gainmap data
• uses SRGB texture so WebGL1 support not guaranteed (but we're sunsetting it soon anyway)
• resizing workflow without third party need clarifications
• jpeg only for now
• jpeg quality when exporting also yields further size decrease (huge drop in size from quality 1 to 0.9, possibly in the 5MB to 500kb) with little compromise on the env map quality.
• I'd recommend we also agree on hdr file naming, including jpeg quality in the file naming: source-name-size-quality.ext eg. polyhaven-brown_photostudio_02-4096x2048-q90.jpg

Notes:

# Get jpeg image quality
identify -format '%Q' file.jpg

from pex-renderer.

can i resize the jpg and keep metadata

No. You need to resize .hdr e.g. in Pixelmator and generate new gaim map

from pex-renderer.