brushlib: Allow MYPAINT_TILE_SIZE to be set by buildsystem
brushlib: Allow MYPAINT_TILE_SIZE to be set by buildsystem

Allows external consumers to potentially use another tile size
then the default 64x64.

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