Fix sorting of QSGTextMaskMaterial (native glyph drawing).

It seems we were hitting the bool operator rather than the
pointer values so all text materials, regardless of size
or style would return equal.

Change-Id: I8773e5aa965035f6737920747c2cf073929ff799
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>