Although very unlikely, the wl_compositor version might not support
wl_surface.set_buffer_scale while the wl_output emits a wl_output.scale
that is larger than 1. So for correctness, bail on changing the buffer
scale if we won't be able to set it later.
Windows now keep track of the monitors they are on, so we can calculate
the best scaling factor for them, by using the maximum of each of the
monitors.
The compositor scales down the buffer automatically when it is on a
lower density monitor, instead of the previous way where it was scaling
up the buffer on higher density monitors, which makes the application
look much better on those ones.
