Support wp_viewporter

[dos1]

Several clients such as SDL or Waydroid are now using viewporter protocol in order to crop and/or scale the attached buffer. It's also the recommended way for clients to deal with fractional output scaling.

Mir appears to be missing support for this protocol entirely.

Weston comes with a simple client to verify the implementation: weston-scaler.

[RAOF]

wp_viewporter is a sensible thing to support - a lot of overlay hardware has capabilities that map very directly onto the interface exposed by wp_viewporter, so this provides optimisation opportunities that would otherwise not be available.

That you can use it to kinda implement fractional scaling is somewhat of a side effect; ideally fractional scaling would be negotiated directly, with the compositor's explicit participation, rather than worked around with a protocol that can be made to sorta get the right effect.

[JamiKettunen]

Mir v2.11's missing wp_viewporter support is now visible on https://wayland.app/protocols/viewporter#compositor-support, when support is added (and new version tagged) https://github.com/vially/wayland-explorer should get a follow-up PR by running wlprobe against mir_demo_server and replacing the existing src/data/compositors/mir.json ^^

[AlanGriffiths]

Closed by #3445