Optimize gamma correction;
Optimize gamma correction;

Currently, we gamma correct colors on every clear and every draw.
It's taking a lot of time.  Instead, we'll gamma correct colors
when they're changed using lovr.graphics.setColor/setBackgroundColor.

bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr