Logic for the recently included fractional scaling support [0] was
difficult to reason about as it depended on two different callbacks
one that listened to a preferred scale, another that tracked which
physical displays the window overlapped.
Checking if fractional scaling was in used depended on the order
the callbacks ran - which is undefined.
In practice - mixing non-fractional and fractional displays would
flicker when the window was moved between monitors.
Resolve this problem with the following changes:
- When the fractional-scale manager is supported,
only respond to the scale from it's preferred_scale callback.
- When no fractional-scale manager is available,
set the scale based on the scale of overlapping outputs.
- Add support for postponing the buffers commit call to prevent
flickering when changing the windows scale.
Other changes:
- Use a lock before setting the pending frame state from
wp_fractional_scale_handle_preferred_scale.
- Ensure pending actions that themselves trigger pending actions
run in the time gwl_window_pending_actions_handle is called.
- Rename GWL_Window::scale -> GWL_WindowFrame::buffer_scale.
[0]: cde99075e8
648417bfff