protocol: Support scaled outputs and surfaces

This adds the wl_surface.set_buffer_scale request, and a wl_output.scale
event. These together lets us support automatic upscaling of "old"
clients on very high resolution monitors, while allowing "new" clients
to take advantage of this to render at the higher resolution when the
surface is displayed on the scaled output.

It is similar to set_buffer_transform in that the buffer is stored in
a transformed pixels (in this case scaled). This means that if an output
is scaled we can directly use the pre-scaled buffer with additional data,
rather than having to scale it.

Additionally this adds a "scaled" flag to the wl_output.mode flags
so that clients know which resolutions are native and which are scaled.

Also, in places where the documentation was previously not clear as to
what coordinate system was used this was fleshed out.

It also adds a scaling_factor event to wl_output that specifies the
scaling of an output.

This is meant to be used for outputs with a very high DPI to tell the
client that this particular output has subpixel precision. Coordinates
in other parts of the protocol, like input events, relative window
positioning and output positioning are still in the compositor space
rather than the scaled space. However, input has subpixel precision
so you can still get input at full resolution.

This setup means global properties like mouse acceleration/speed,
pointer size, monitor geometry, etc can be specified in a "mostly
similar" resolution even on a multimonitor setup where some monitors
are low dpi and some are e.g. retina-class outputs.