backend-wayland: 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 other renderers in the future.

Also makes sure that weston --backend=wayland --renderer=noop fails
with an error message instead of segfaulting.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>