xwayland: Use the size in wl_output in mode

A mode represents the underlying hardware. The software transformation
information is delivered via RRTransform and reflected to RRScreen.

v2:
- Use mode size for Wayland viewport functions (Olivier Fourdan)
- Introduce new names, mode_width and mode_height

v3:
- Call RRCrtcSetTransformSupport (Michel Dänzer)
- Fix transform argument of RRCrtcNotify
- Rename x and y to logical_x and logical_y

Signed-off-by: Akihiko Odaki <akihiko.odaki@gmail.com>