Fix #34817: improve OpenGL preview render speed, it got quite a bit slower since 2.60.

Doing linearization with GLSL was already faster, but even faster is to just read the
bytes instead of floats and convert those to linear, since byte => float is just a quick
256 entry table lookup. Also made it assign the bytes directly to the image buffer so
they do not need to be converted back from float to byte for file saving, and made sky
render write the background color with OpenGL instead of doing it on the CPU.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55970 954f8c5b-7b00-dc11-b283-0030488c597c