This function is only used in Sequencer code to convert source byte images
into float images. For a "simple case" where there is no color space
conversion needed, it was doing that on a single thread, in two passes over
the image (first byte -> float conversion, then alpha premultiply).
VSE 4K resolution playback, with two image strips where one of them has
"convert to float" option, on Windows / Ryzen 5950X:
- Overall playback FPS: 3.40 -> 4.03. Still very slow, and a lot of time
is spent in allocating, clearing and deleting various temp images, as
well as final color conversion for display.
- `seq_imbuf_to_sequencer_space` part: 217.9ms -> 86.6ms. Most of
remaining cost is just in just memory allocation+clear, which is why
it's not drastically faster due to threading.
Pull Request: https://projects.blender.org/blender/blender/pulls/115628
f3ce0645e4