backend-rdp: extract weston_output_set_single_mode()

The ensure_single_mode() helper replaces an output's single mode.
Extract it into libweston so it can be reused by the VNC backend,
and rename it to weston_output_set_single_mode().

At the same time, set the the previously missing
WL_OUTPUT_MODE_CURRENT flag on the new mode.

Fixes #758

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