Add possible capabilities CURSOR_PLANE and ARBITRARY_MODE

Add possible capabilities CURSOR_PLANE and ARBITRARY_MODE

The CURSOR_PLANE capability indicates that the backend has a concept of a
cursor plane and can handle a cursor without compositing.  This is currently
only advertised by the DRM backend.

The ARBITRARY_MODE flag specifies that the backend is capable of switching to
virtually any resolution.  This is currently only advertised in the RDP
backend.  While it's a bit buggy right now, it should be capable of this.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>