Backport of "compositor: fix blending of subsampled components" into 1.18

Backport of !988 (merged) "compositor: miscellaneous blending fixes".

Todo:

• Needs manual intervention, cherry-picking failed!

Commits:

• 8ff5079e compositor/blend: fix blending of subsampled components
• 247f8218 compositor/blend.c: fix MT fill_checker for YUY2 (only relevant for multi-threaded blending which is only in master not in 1.18)
• e0a4d3ac compositor/blend.c: fix MT checker pattern (only relevant for multi-threaded blending which is only in master not in 1.18)