When a strip with alpha-over blend mode (which is default) has an alpha
channel and is at the bottom of the whole stack, there's no point in
blending it with black color; the result will be identical.
So stop doing that. Also, whenever some other case happens at the bottom
of the stack (e.g. some other blend mode), create the fake "black input"
image at the needed color type (float or byte) and with the correct
color space already applied. Otherwise, especially for floats, VSE
spends much time converting this fake black input from byte to float,
and then converting it to sequencer color space.
Test case of two EXR files blended over each other, at 4K resolution,
on Ryzen 5950X (Win10/VS2022):
- whole sequencer_preview_region_draw 115ms -> 75ms
- seq_render_give_ibuf part: 64ms -> 24ms
Pull Request: https://projects.blender.org/blender/blender/pulls/127310
5c259a1bae