Avoid infinite loop with distance fields disabled

createGlyphNode() and createNativeGlyphNode() kept calling each other
on GLES whenever QML_DISABLE_DISTANCEFIELD was set.

Change-Id: Ic1c2cfe0c4c7301f82cbbcce1cb512bd515b52ef
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>