Fix 2542

[pwuertz]

This PR fixes #2542 by first adding the capstyle and joinstyle properties to matplotlib.patches.Patch and setting the default capstyle of the axes spines to 'projecting'.

[mdboom]

Great. Would you mind adding or modifying a regression test that does something like what your image example above does?

[pwuertz]

I've got a problem with the image comparison tests. Two of the low-dpi image tests are failing due to an artifact from the pdf to png conversion process. On the left you see the PDFs with and without the capstyle modifications. On the right are the PNGs produced by the image comparison test.

I suppose making changes to the PDF comparison toolchain is a bit out of scope. Although they look odd, shall I just commit the new images as test reference for now?

[pwuertz]

@mdboom: I think this PR is ready to merge if you are ok with it.

I modified an existing test to workaround PDF rendering issues, updated some baseline images and added a new test for the new spine capstyle. The commits are rebased to the current master.

[mdboom]

Thanks so much for getting to the bottom of this. This is awesome.

[tacaswell]

Should this be cherry-picked to 1.3?

[pwuertz]

There is no real API change, so probably yes.