The recently introduced size, strength, and jitter pressure curves
affect most paint code. This exposed further odd behavior inside
`paint_brush_update` where the size pressure curve was being evaluated
even if the brush's size did not vary with pressure.
To fix this issue, this commit clarifies a few comments and updates the
code flow such that cached input values and evaluated pressure values
are used more consistently.
Pull Request: https://projects.blender.org/blender/blender/pulls/148077
fd3d9f2fa2