Support oblique and emboldened glyphs in SceneGraph

When the font does not have an italic or bold variant, we should artificially embolden or slant the glyphs. This is especially important for scripts with large glyph sets, like Chinese, where font variants are less common. Currently this is not supported at all in SceneGraph, so it's a regression against Qt 4. Synthesized oblique currently looks awful, because the glyph metrics are processed but not the glyphs, which means the glyphs will just be stretched instead.

The easiest solution might be to have the font engine support this, by applying the transformation to the outline before returning it (which would also fix QTBUG-20817.) That would also allow a back-end specific implementation.