Blender Git Loki

OpenGL render: Avoid byte-float-byte roundtrip

It was annoyingly slow to do roundtrip from byte OpenGL render to
float render result and back to byte image format (which is used
in 99% of cases for the OpenGL previews),

Now we use render result's rect32 to store render result which is
already supposed to be in the display space.

Gives about 30% speed improvement for OpenGL previews here.