Count the uses of paths in a collection

Add a way for backends to query how many times each
path in a collection can be reused.
Use the uses_per_path data in pdf backend

Fixes matplotlib/matplotlib#3345
Add the new test in the list of tests
Add new baseline images for two tests

These look closer to their png counterparts than the old ones.
There probably is a remaining bug in the drawing of edges.
Handle path collections without stroke in PDF backend
Merge pull request #3 from mdboom/pdf-xobject-linewidth

Pdf xobject linewidth
Reduce the emitting of object definitions in ps and svg

The cost calculations are very rough, back-of-the-envelope.
In the example from #3345 I get significant file size reductions:

	2.3M	before.svg
	1.8M	after.svg
	916K	before.ps
	672K	after.ps
Merge pull request #3485 from jkseppan/reduce-xobjects

ENH : Reduce the use of XObjects in pdf backend