Improve rounded border corners

[WillPower3309]

• Description:
  There are some improvements that could be made in how we handle rendering rounded corners. Currently, rounded borders are done by drawing circles and removing the parts that are not along the corner we are currently rendering, and rounded titlebars will be blended with a window's rounded corners through its fragment shader, which is less than ideal for keeping things simple. I'll be working on making these things cleaner (and hopefully more performant), once we get an initial release out.

[WillPower3309]

still need to do this, but for now I've cleaned up the existing border shader from 54 to 36 lines

[WillPower3309]

Related:

• Rendering optimization: store rounded corners in texture to use 1 quad per corner ocornut/imgui#1962
• https://github.com/Javyre/swayfire/blob/master/src/deco/subsurf.cpp#L19-L52
• https://stackoverflow.com/questions/20508259/draw-a-curved-line-from-an-arc-edge

[ErikReider]

Oh! That looks promising!

[WillPower3309]

If we go the stencil route, this would likely fix #113 and #47 (and integrate well with #127)

[WillPower3309]

Will be fixed once we use the scenefx border impl in wlrfx/scenefx#59