wayland: Add support for the fractional scale protocol

wayland: Add support for the fractional scale protocol

April fools!

No, really.
The fractional scale protocol is just a way to track the surface scale,
but not a way to draw fractional content.
This commit uses it for that, so tht we don't rely on tracking outputs.

This also allows magnifiers etc to send us a larger (integer) scale if
they would like that, that is not represented by the outputs.