Blender Git Loki

Cycles Bake: use size_t instead of width, height

(original patch by Sergey Sharybin)

Note: RNA API can't use size_t at the moment. Once it does this patch
can be tweaked a bit to fully benefit from size_t larger dimensions.
(right now num_pixels is passed as int)

Reviewed By: sergey, campbellbarton
Differential Revision: https://developer.blender.org/D688