brushlib: Split out more code in operation queue that manipulates tile map
brushlib: Split out more code in operation queue that manipulates tile map

Also marks the know issues the code has. Will be fixed shortly.

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