brushlib: Update gegl.py example with recent progress
brushlib: Update gegl.py example with recent progress

Can now instantiate and draw onto a MyPaintGegl.TiledSurface,
and it should be possible to hook this up to a GeglGtk.View
for rendering.

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