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RenderViewport: Texture Format

When doing viewport rendering the color management happens on the CPU.
This has overhead in downloading a float texture from the gpu and
performing color management on the CPU.

Based on the scene fileformat bit depth the result will be rendered to
a byte texture where the colormanagement happens on the GPU or a float
texture where the colormanagement happens on the CPU.

This is only done during `Viewport Render Animation` in other
cases a float texture is being used.

Baseline (HD render of wanderer.blend workbench engine no samples) 15.688038 s
After changes: 9.412880s

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D6195