Appart from the color glitch, there was several problems with vpaint: * "fast_update" mode was never on, because of wrong testing code; * drawing refresh during stroke in "fast_update" (i.e. no dm rebuild) mode was broken in VBO mode, because updated (tess data) mcol wasn't moved to colors GPUBuffer. Solved the later point by adding a new DM_DIRTY_MCOL_UPDATE_DRAW flag to DerivedMesh dirty var, which is set each time vpaint stroke directly update me->mcol, and forces GPU_color_setup() to refresh the gpu's colors buffer. Also got rid of the uggly GPU_color3_upload(), which basically did the same thing, but with an additional intermediate buffer !