Reduce coupling between _tkagg and _backend_agg modules
Merge pull request #3778 from mdboom/tkagg-simplify

MNT : Reduce coupling between _tkagg and _backend_agg modules
Upgrade Agg to SVN r103
Workaround for blending bug in new Agg.
Make cell_block_limit non-hard-coded
Move typedefs

Conflicts:
	src/_tkagg.cpp
Move Agg directory

Conflicts:
	setupext.py
Use Agg workarounds in image module as well
Update tests to more accurate results