Skip gamma correction of cursor data if premultiplied R/G/B > alpha

The un-premultiplied R/G/B values would overflow the gamma LUT, so just
pass through the data unchanged, and leave it up to the HW how to
interpret such weird premultiplied alpha pixels.

11 jobs for cursor-unpremultiply-overflow-no-gamma in 2 minutes and 53 seconds (queued for 19 seconds)