When creating a new display list the pango renderer tries to add a
callback to the glyph cache so that it can be notified whenever the
atlas is reorganized. However it was always registering the callback
for the glyph cache which doesn't use mipmapping so if mipmapping is
enabled then it wouldn't work correctly.
This patch moves the two sets of caches (pipeline cache and glyph
cache) into one struct so that it's a little bit easier to determine
which pair to use in the code.
