wayland: Add basic hidpi support

Advertise the scale factor on the output and transform pointer and damage
events as well as input and opaque regions for clients
that scale up by themselves i.e use set_buffer_scale.

We do not scale any 'legacy' apps yet.

https://bugzilla.gnome.org/show_bug.cgi?id=728902
wayland: Scale native surfaces for hidpi

Scale surfaces based on output scale and the buffer scale set by them.
We pick the scale factor of the monitor there are mostly on.

We only handle native i.e non xwayland / legacy clients yet.

https://bugzilla.gnome.org/show_bug.cgi?id=728902