When rendering a box for an unknown glyph it would previously just use
the average glyph size for the font. This causes problems because the
size calculations for the layout assume a different size so it can end
up rendering outside of the expected ink rectangle. This patch changes
it to query the size of the glyph in the font. Pango should end up
reporting the size of what would be the hex box which should be the
same as the sized used for the extents calculation.
http://bugzilla.clutter-project.org/show_bug.cgi?id=2599
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.
