wayland/background-effect: add ext-background-effect-v1 (blur region)

[bbedward]

core/region: add radius to create rounded regions

Adds BackgroundEffect that allows setting blur region on layer shell surfaces, additionally adds a radius option to Region similar to the vicinae pattern for making approximate rounded regions.

To test on a supported compositor: qs -p src/wayland/background_effect/test/manual/background_effect.qml

Usage is as:

BackgroundEffect.blurRegion: Region {
        item: root.contentItem
}

[YaLTeR]

Seems the region passed to the protocol also needs to be offset by the Qt CSD, if any:

Better visibility of the region:

[outfoxxed]

For consistency with existing interfaces, this should probably be an attached object. See HyprlandWindow for an example.

Do we have any options for blur opacity exposed?

[bbedward]

For consistency with existing interfaces, this should probably be an attached object. See HyprlandWindow for an example.

Changed it to an attached object, the only downside I guess is my thought was getting a BackgroundEffect.active to tell if the compositor had applied it/supported it

Do we have any options for blur opacity exposed?

That's not exposed by the protocol, its just blurRegion and capabilities { blur } pretty much. The effect parameters themselves would be set elsewhere, like niri's compositor config in this case.