RFC: Sunsetting XRender backend

Hi,

The Xrender backend was added at the time when OpenGL drivers were not 
particularly known for being rock solid stable. So, having Xrender 
render backend as fallback did make a lot of sense back then. Nowadays 
though, it's not the case. OpenGL render backend has been the default 
one for many years. It's quite stable, and it allows implementing more 
features, e.g. wobbly windows effect, etc. Given that, the Xrender 
render backend is in a bitrotten state now. Many features are not tested 
with it; the only time when it gets noticed is when changes in other 
parts of kwin break build in the xrender backend.

Another issue is that we would like to provide better integration 
between kwin's scene graph and qtquick's scene graph. QtQuick doesn't 
support and most likely will never support Xrender. This poses a problem 
as we would like to provide native thumbnail item support in qtquick and 
re-implement effects such as desktop grid and present windows using QML.

Given the bitrotten state, our focus being shifted towards wayland, and 
no native Xrender support in qtquick, I propose to remove the xrender 
backend. Thoughts?

Cheers,
Vlad