Despite my previous commit here, the time was still being spent building
bound rect's for font glyphs, so improve the caching so we don't need to
clear it when we change the font selected into the DC.

Change-Id: Iee8230fc76d9d809f3521d016e4ce9a6555e6f65
Reviewed-on: https://gerrit.libreoffice.org/53371Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>
Tested-by: Noel Grandin <noel.grandin@collabora.co.uk>