Handle the emit screenshot headless command.

But, it doesn't work.  Shouldn't the vram have the graphics?
Simple Windows-only headless screenshot compare.
Bitmaps are stored BGR, so let's read it that way.
Separate out the actual screenshot comparison.

Fixes #109.
Merge pull request #621 from unknownbrackets/screenshot

Headless screenshot support
Merge remote-tracking branch 'upstream/master' into fps2