Decouple QSGDistanceFieldGlyphNode from it's cache manager.

To implement a custom distance field glyph node currently it's
necessary to also provide a duplicate implementation of
QSGContext::createDistanceFieldGlyphCache() as the default implemention
references the cache manager created by createGlyphNode().  By isolating
references to the cache manager to just createDistanceFieldGlyph() cache
it becomes possible to just overwrite createGlyphNode() and still use
the default cache.

Change-Id: I7261bdbf247966b55512d2671e2ee85239bcca05
Reviewed-by: Yoann Lopes <yoann.lopes@nokia.com>