With raster a QPixmap is no longer a XPixmap which fails all code
which assumes that an QPixmap is an XPixmap. Depending on were in
the codebase we either convert such pixmaps to images (OpenGL) or
create a XPixmap and use QPixmap::fromX11Pixmap to get a "real"
pixmap.

It is possible that there are more code pathes were we would need
a XPixmap. Currently tested is basic functionality of no-compositing,
XRender compositing, OpenGl/GLX and OpenGL ES/EGL compositing.

For OpenGL compositing raster might result in performance improvements,
for XRender it is possible that there are regressions when using raster.
By default KWin uses whatever is the default of the system, so we just
no longer enforce native.

Of course it is a bad idea to use graphicssystem OpenGL. As that
is broken anyways in Qt, we do not check for it.

Many thanks to Philipp Knechtges for bringing up the issue, convincing
me that we need it and providing most of the patch.

REVIEW: 101132
CCMAIL: Philipp.Knechtges@rwth-aachen.de