Add new blend effect

When changing global settings like switching between light and dark
themes the result is currently quite messy. Apps update in a scattered
order and a jarring change.

The solution is heavily inspired from Gnome, we cache the window for X
milliseconds, then perform a crossfade between the cache and current
contents.

It does mean any video is paused for half a second, but pragmatically
it's not really bothersome.

On receipt of an explicit DBus method call we create a cache and start
animating. The explicit method call is needed so that we can guarantee
kwin has a fully copy before a client might change.

Use of an offscreen texture is used so that we include any window
decorations. The DeformEffect effect is repurposed as it has most the
relevnat logic if we remove the live updating.