Filling the background Box of a text strip was a single-threaded
code before. Which was pretty fast for a simple rectangle, but
when roundness is used it becomes a bit slower (super-ellipse
equation has to be evaluated for each pixel in the rounded corners,
which is 3x powf per pixel).
So make the background box use multi-threading. On M1 Max, filling
background box of 2256x1691 pixels:
- No roundness: 25.8ms -> 4.3ms
- Roundness 0.3 (253 pixels): 31.9ms -> 5.8ms
- Roundness 1.0 (845 pixels): 94.6ms -> 15.8ms
Pull Request: https://projects.blender.org/blender/blender/pulls/130403
b26dc3dd01