We were using the screen physical size to determine if a screen is
portrait or landscape, but we may not always get a valid size.
Instead use the screen resolution.
The orientation is then calculated in the wl_output.done handler,
since we don't know the resolution yet when receiving the first
wl_output.geometry event.

Change-Id: I3554f916e54db829f49fa3d1ea24f7ce1ff24e7c
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>