This was previously done only in the default cache implementation. It
has been moved to the base class.
releaseGlyphs() is called when a glyph is not referenced anymore by any
node.
Added a virtual function referenceGlyphs() that is called everytime glyphs
are being used in a node. This function is called just before
requestGlyphs().

Change-Id: If90f86c328c18ae2a5977847a6adf50b99ea1241
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>