Fix Text layout cache lookup.

Even though `Text._get_layout` takes a `renderer` parameter, the cache
is looked up by the `self._renderer` attribute. If an alternate renderer
is provided without changing any other properties, the cached layout
from the previous renderer is returned.

An alternate renderer is passed by `offsetbox.TextArea`, which, through
its use in legends, causes the legend to shift slightly if figures are
saved in different formats.

Fixes matplotlib#6899.

Author: QuLogic