Fix part #33935: Texture painting slow down with mouse, but not with tablet

Issue is solved for painting on byte buffer with default sRGB display enabled.
In this case it is possible to skip any color space transform and just apply
dither if needed.

Still not sure if there's a regression in painting on flaots or not, will
continue investigation.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54381 954f8c5b-7b00-dc11-b283-0030488c597c