backends/x11: Restore dedicated XRenderUtils::init function

Under kwin_wayland `kwinApp()->connection()` is for communicating with
XWayland, but in X11Windowed backend we need to talk to the host XServer.

Restore `XRenderUtils::init` and set it accordingly based on
whether we're running standalone or windowed, so that `kwin_wayland`
works running nested in an X session again.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>
Tested-by: Merge Service <!2457>
Part-of: <!2457>