Reference count glyphs in QSGDistanceFieldGlyphCache.

Reference count glyphs in QSGDistanceFieldGlyphCache.

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>