previous fix for 2D image painting were done, and also:
* For brushes that do masking (keeping the max contribution to a pixel limited),
the alpha from textures is now part of the mask. In many cases the logic
worked out the same and where it didn't it used to cause artifacts.
* Color interpolation for smear and soften tools now happens in premultipied
space, to avoid bleeding of RGB colors from zero alpha areas.
* Fix a few cases where byte <=> float conversion was not doing the proper
straight <=> premul conversion.
* Replace some float division by constants with multiplications, is faster.
Note: float texture painting seems to have some issues updating the OpenGL
texture, but issue was already there before this commit.
fe6c45e36e