Viewport enhancements

[McManning]

Ideas for enhancing performance:

• The VP the user is actively working in within Blender should get full resolution / sync priority. Additional viewports would be updated slightly less (I'm talking 60 FPS -> 30 FPS, so still reasonable speeds) while also getting a lower resolution texture to reduce copy effort (e.g. 75% or 50% resolution)
• Try packing as BGRA/RGBA in transit. Some GL articles claim that there's a performance boost this way (since the driver doesn't need to do any conversions during upload) - but we gain ~2 MB/frame overhead for all copy ops between apps