IMB_alpha_under_color_[byte/float] functions are used when preparing
the rendered image for image/movie output with RGB channels (i.e. no
transparency). They were single threaded before, multi-thread them.
Time taken by them on 4K resolution image (mix of various transparency
values in source), on Ryzen 5950X/Windows:
- IMB_alpha_under_color_byte: 10.1ms -> 1.9ms
- IMB_alpha_under_color_float: 14.6ms -> 8.8ms (smaller speedup since
it becomes memory bandwidth limited)
Pull Request: https://projects.blender.org/blender/blender/pulls/135258
a33730dc0d