frontend: Introduce 'same-as' key for configuring overlapping outputs
Patch series introduces 'same-as' keyword for configuring overlapping outputs/sharing of native outputs.
The first patch fixes RDP backend to use simple_heads_changed, and to allow
a uniform configuration across all the secondary back-ends.
The last patch documents it and it makes a clear distinction between this same-as and and the one from the DRM backend.
A limitation of this patch series is that it doesn't create a virtual output that spawns spans
over all the available outputs, so with this is can be used to share a particular
native output or 'none' to align the virtual output to the right.