Blender Git Loki

replace masking rasterizer with a more simple geometry based rasterizer (for the compositor).

notes:
- uncomment #define USE_RASKTER in BKE_mask.h to use the previous mask rasterizer.
- slightly slower for regular masks but significantly faster for feather.
- main benefit is that it threads well so works nice for tile compositor.
- feather is lower quality, can use some improvements here.
- feather can also use some interpolation enhancements, will do later.