weston: Port RDP backend to new output handling API

weston: Port RDP backend to new output handling API

This is a complete port of the RDP backend that uses
the recently added output handling API for output
configuration.

Output can be configured at runtime by passing the
necessary configuration parameters, which can be
filled in manually or obtained from the command line
using previously added functionality. It is required
that the scale and transform values are set using
the previously added functionality.

After everything has been set, output needs to be
enabled manually using weston_output_enable().

v2:

 - Rename output_configure() to output_set_size()
   in plugin API and describe it.
 - Manually fetch parsed_options from wet_compositor.
 - Call rdp_output_disable() explicitly from
   rdp_output_destroy().

v3:

 - Disallow calling rdp_output_set_size more than once.
 - Manually assign a hardcoded name to an output as that's
   now mandatory.
 - Use weston_compositor_add_pending_output().
 - Bump weston_rdp_backend_config version to 2.

Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>