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.
Add a new flag to track whether buffer scale is dirty or not,
and centralize calling wl_surface_set_buffer_scale() in a single
place: gdk_wayland_surface_sync_buffer_scale().
gdk_wayland_surface_sync_buffer_scale() is only called by
gdk_wayland_surface_sync(), which itself is called by the GL,
Vulkan, and Cairo contexts, right before submitting a frame.
This ensure that each frame has an up-to-date buffer scale.
This mimics how opaque and input regions are tracked.
