backend-x11: Prepare for more renderers, reject no-op

Turn the Pixman/GL if/else conditionals into switch cases to make it
easier to add support for others renderer in the future.

Also makes sure that weston --backend=x11 --renderer=noop fails
with an error message instead of starting with the GL renderer.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>