brushlib: Flush queued draw_dab operations on get_color
brushlib: Flush queued draw_dab operations on get_color

The result of get_color, which must be returned to the brush engine
immediately may depend on the draw_dab operations for the affected tiles.
Also add some comments about where we can use OpenMP concurrent processing.

b3sigma/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,achadwick/libmypaint