x11: Make sure that outputs are stored in xinerama order

With the new output query algorithm, position of an output may not align with its xinerama index. This can be problem on x11 as some protocols, e.g. startup feedback, use xinerama indices to identify outputs.

Technically, the RandR specification doesn't say that CRTCs are stored in the xinerama order, it only mentions that the first CRTC corresponds to the primary output. However, such assumption was made by kwin prior to output query changes, this change merely restores that behavior.