Added support for blur effects

[xiaozhuo256]

Similar to swayfx's objectives, add more visual appeal.

[dawsers]

SwayFX uses its own scene graph and renderer. Scroll uses wlroots's scene graph, but modified to support content and workspace scaling. After #3 there is an effort by the author of the issue report to merge both and create ScrollFX.

On my side, there are no current plans to implement any blur effects. For that, I would need to write a new renderer, and that would separate the code base even more from wlroots and sway. wlroots's main renderer is still GLES2, and there are plans to move to Vulkan as main target. Rewriting a GLES2 renderer when it will not be the main one soon feels like a wasted effort to me.

Implementing blur has a lot of side effects. It needs multiple rendering passes, affects damage tracking and clients would need a way to notify if they are applying some alpha effect or it wouldn't work well. It also makes performance worse, and it is very prone to bugs because there is no Wayland standard for clients. There are some Wayland extensions coming up to fix some of the issues, but things are not standard enough for me to go that route at this time.