Blender Git Loki

Use undo tile colours and masks as projection pixel original colours and
accumulation masks respectively. This will solve artifacts when painting
on meshes where two faces reference the same region in UV space. Of
course, painting on a multithreaded system is bound to create some
conflicts if you paint with a brush that will put different colours on
the same region (for instance a textured brush), but at least you don't
get the final colour reset to the original due to a pixel still having a
zero accumulated mask in screen space.

For more correct results, synchronization (locking) on accululated mask
writing should occur but I don't think this is too crucial and it will
hurt performace somewhat if it is added. More tests are needed to assert
that it is indeed not necessary.