Allow applications to opt-out of OpenGL initialization

For applications that do not require OpenGL, it would be nice to allow them to opt-out of initializing the Mesa driver, since the initialization can be computationally expensive. QApplication already has AA_ForceRasterWidgets, which causes the use of plain RasterSurface everywhere instead of RasterGLSurface. It is a trivial change in the XCB backend to check for a RasterSurface (in qxcbwindow.cpp) and avoid the OpenGL initialization path in this case.

As an example, Audacious shows a 23% reduction in startup time (as measured by Valgrind) with this patch.