Background dithering was introduced to solve banding issues on gradient backgrounds.
This patch will enable dithering based on the texture that is used for drawing.
Only when using a GPU_RGBA8 texture the dithering will be enabled.
This disables dithering for final rendering, vertex and texture paint modes.
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D6056
b0476f0631