QWidget: Enable renderToTexture mode for all widgets

aka QOpenGLWidget mode without the QOpenGLWidget.

This is useful for at least two cases:

1) QTBUG-49856: On OS X in layer mode we need to create either an OpenGL or a Raster layer. Handling RasterGLSurface then becomes difficult. We can resolve that by always rendering using OpenGL when in layer mode.

2) QTBUG-50991: Support fractional scale factors in high-dpi mode. QtWigets renders to a texture at an integer scale factor, OPenGL is used to scale that texture down to the target fractional scale factor.

API:
The platform plugin would enable this per-window, for example via QPlatformWindow API:

   bool useOpenGlRenderingOnlyForRasterGLSurface() const