New TransformedPatchPath caching object

A newly added :class:`~matplotlib.transforms.TransformedPatchPath` provides a

means to transform a :class:`~matplotlib.patches.Patch` into a

:class:`~matplotlib.path.Path` via a :class:`~matplotlib.transforms.Transform`

while caching the resulting path. If neither the patch nor the transform have

changed, a cached copy of the path is returned.

This class differs from the older

:class:`~matplotlib.transforms.TransformedPath` in that it is able to refresh

itself based on the underlying patch while the older class uses an immutable

path.