More distance-field cache refactoring.

The distance field glyph node now uses preprocess().
The glyph cache is updated at that time, then when all the glyphs are
ready the node's geometry is updated.

addGlyphPositions and addGlyphTextures in QSGDistanceFieldGlyphCache
have been renamed to setGlyphsPosition and setGlyphsTexture to reflect the
fact that they can be used to update existing glyphs. For example when
a glyph has moved to a different texture.
When an existing glyph is updated, all nodes containing that glyph are
invalidated and their geometries are reconstructed with the new values.

Change-Id: I7758313155f48811e6027434e6c9a1c3df5dfab7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>