The `Use Uniform Width` option did not work the intended way.
Like the comment and the manual says it should "enforce uniform
stroke width by averaging radius".
Instead (as far as I understand) it was simply checking if the stroke had
radii that were all within some margin from one another and then
returning the first radius if that was the case.
There were two other issues:
1. The widths were not computed correctly, multiplying the pixel size
instead of dividing it.
2. The epsilon to compare the radii was a bit too high (factoring in the
first issue, this actually never returned a value).
This now computes the widths in screen space and then returns the mean.
Pull Request: https://projects.blender.org/blender/blender/pulls/136903
ac201c2d9a