staging: add ext-blur-v1

This protocol allows the client to specify a region behind the surface that should be blurred, with the intention to improve the visuals of for example panels or terminals.

This protocol is based on the org_kde_kwin_blur protocol, which has been in use since 2015.

Signed-off-by: Xaver Hugl xaver.hugl@kde.org

The changes vs !43 are:

• I adjusted the naming and "unstable" disclaimer to how newer protocols are done
• The object becomes inert when the surface is destroyed, instead of having an error for that case
• the initial region is empty, which is more similar to how other protocols deal with this sort of extension (=don't change anything by default)

Merge requirements:

• 2 ACKS
• one compositor implementation: https://invent.kde.org/plasma/kwin/-/merge_requests/4890
• one client implementation