wayland: Implement fractional_scale_v1 protocol

Giving clients hints about optimal fractional scaling ratios,
to be used together with the `wp_viewport` protocol.

See
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/blob/4624cfaaf563cd7be5e2e2087c8de6d3a48ea867/staging/fractional-scale/fractional-scale-v1.xml