shaders/roundedcorners: base on kde-rounded-corners' shader

[taj-ny]

This should make rounded corners look exactly like KDE-Rounded-Corners' corners. Currently they don't scale properly (on this branch, not main or v1.3.2).

Closes #34.

[taj-ny]

This is what corners look like as of the latest commit. The application is Konsole with a light theme. Static blur with a red image was used to make any differences visible.

10px radius, 80% background transparency:

21px radius, 80% background transparency:

10px radius, 20% background transparency:

21px radius, 20% background transparency:

10px radius, 0% background transparency:

21px radius, 0% background transparency

Anti-aliasing can be adjusted. A high value results in sharp corners on high transparency (first image), and the blur barely sticking out on low transparency (last). A low value results in smooth corners on high transparency, and the blur sticking out more on low transparency. Since this effect doesn't have access to the alpha of each individual pixel of the window, there's nothing I can do to improve this.