Makes QSGContext::createGlyphNode() the central point of determining which
glyph node to produce, instead of letting the caller call two different
versions of the factory, each one calling the other in various cases and
behind various ifdefs.

Change-Id: I30fb17cceab45d9e13ddf3ece7a65f220c5e5acd
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>