Enablers for using QGlyphRun in SceneGraph

Enablers for using QGlyphRun in SceneGraph

Several required fixes and changes to fix problems that arose
when porting SceneGraph's QSGTextInput and QSGTextEdit to
use QSGTextNode, especially related to having selections
on the text.

Also fixes crashes with the threaded renderer on Mac OS X
when using the TextEdit or TextInput elements.

Task-number: QTBUG-18019, QTBUG-20017
Change-Id: I67f24465352daa1d2cb12b6d2f378feb676c9804
Reviewed-on: http://codereview.qt.nokia.com/2864
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>