Fix material compare for distance field glyph nodes.

The compare function did not take into account the GL texture ID
when comparing materials. This could result in a renderer
merging glyph nodes into a single batch incorrectly.

Change-Id: Ib62c43f93fb1bbbc231197323dced4254ffa12aa
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>