Disable support for RasterOpModes in GL paint engine

This leads to crashes for instance when displaying a text cursor
in a graphics scene.

Change-Id: I1b5c884ddb8325a7f5bdbc6027f0fae13f139a1c
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/opengl/qopenglpaintengine.cpp

@@ -1330,6 +1330,7 @@ void QOpenGL2PaintEngineExPrivate::drawVertexArrays(const float *data, int *stop
 QOpenGL2PaintEngineEx::QOpenGL2PaintEngineEx()
     : QPaintEngineEx(*(new QOpenGL2PaintEngineExPrivate(this)))
 {
+    gccaps &= ~QPaintEngine::RasterOpModes;
 }
 
 QOpenGL2PaintEngineEx::~QOpenGL2PaintEngineEx()