This shaves off ~33% runtime from

from pylab import *
import timeit

tx = figtext(.5, .5, "foo\nbar baz")
gcf().canvas.draw()
gcf().patch.set_visible(False)
r = gcf()._cachedRenderer
n, t = timeit.Timer(lambda: tx.draw(r)).autorange(); print(t / n, n, t)

because previously the calls to realpath were not cached and filesystem
access is slow.

(The font cache is likely already in trouble if fonts move in the
filesystem within the execution of a program given that there are other
layers of cache, so I wouldn't overly worry about that. At best we may
want to expose a manual function to invalidate all caches if someone
really runs into that problem.)

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/next_api_changes/* if API changed in a backward-incompatible way