This way the font cache can correctly invalidate the cached glyph
rects when a font is dropped from the cache.

Change-Id: I050866099742334f01cac1b872228a017ddb5e9b
Reviewed-on: https://gerrit.libreoffice.org/61371
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>