ext-blur: New protocol

This patch introduces a protocol that provides a way to specify the area of a surface that needs to have its background blurred by the compositor.

An intended use case is to improve visuals of translucent surfaces such as panels or docks by blurring background behind them.

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

Signed-off-by: Vlad Zahorodnii vlad.zahorodnii@kde.org