Added support for individual rounded corners in the experimental backend

[Philipp-M]

This PR adds support for individual rounded corners, to be able to achieve the effect as discussed in my previous comment: yshui#361 (comment)

Each rounded corner can be configured per window like corner-radius using the new config options corner-radius-top-left, corner-radius-top-right, corner-radius-bottom-right and corner-radius-bottom-left.

Though because the old backends are deprecated someday in the future I haven't added support for them.

[ibhagwan]

Chapeau @Philipp-M, this is a really neat implementation converting u_radius to vec4 and modifying the RectSDF function, gotta love open source :-)

[snoweuph]

soo, are Individual Rounded Corners for Individual Windows are in work?

[Philipp-M]

This is quite old and has diverged quite a bit from the original repo.
It should be rebased onto the original repo (and probably implement this as well for other backends) but I don't have the time to do that, maybe someone else wants to try?

[ibhagwan]

This is quite old and has diverged quite a bit from the original repo. It should be rebased onto the original repo (and probably implement this as well for other backends) but I don't have the time to do that, maybe someone else wants to try?

Most of the functionality (maybe all by now?) of this fork is now merged upstream, if someone wants to do it they should probably rebase this PR on the upstream.