Use multithreading, plus make the non-float / non-separate luma
calculations faster by avoiding byte->float->byte conversions
back and forth.
On a 4K resolution sequencer display, time taken to calculate the
waveform (Windows, Ryzen 5950X):
- regular images: 127.0ms -> 6.4ms
- regular images, separate colors: 160.3ms -> 13.1ms
- float images: 86.2ms -> 11.1ms
- float images, separate colors: 162.9ms -> 17.4ms
This also fixes curious "one black pixel row" in the middle of the
waveform, which was caused by the code spreading 256 possible luma
values over "off by one" vertical range:
Pull Request: https://projects.blender.org/blender/blender/pulls/115579
93ca2788ff