Blender Git Loki

Fix crash when changing and using between layer and other brushes in
dyntopo

Layer brush would not invalidate the layer_disp arrays in dyntopo mode,
checking only for the existence of the array. This means that if a tool
resized the node due to topology changes, the layer brush code could
index (and write!) out of bounds in the array. Solution is to invalidate
the layer data prior to each stroke in dyntopo.