Color banding issues can appear, as result of the 8 bitdepth RGBA that
is used in the viewport.
This change will use `GPU_RGBA16F` for final renderings and for drawing textures. This
allows displaying HDRI textures. Vertex Colors uses `GPU_RGBA16` to resolve
color banding issues. All other modes use `GPU_RGBA8` to reduce
bandwidth and gpu memory.
Reviewed By: fclem, brecht
Differential Revision: https://developer.blender.org/D5179
85c3e12049