Glyph Nodes use preprocess to prepare glyphs even though they do not need changes

For a large amount of glyph nodes, the time the renderer needs to spend in preprocessing nodes for no good reason is quite large. In the case of http://qt.gitorious.org/qtplayground/scenegraph/trees/master/benchmarks/extremetable We're spending up to 4ms in preprocess (measured using QSG_RENDER_TIMING=1).

An alternative approach to preparing the glyphs should be taken. For instance, one manager object could be connected to QQuickWindow::beforeRendering() and it could prepare all "registered" glyphs at this time. That would greatly improve performance for text-heavy apps.