Border fixes (fixes: #399)

[Xgames123]

• fixed border with corner radius not clipping bg
• fixed corners being too thick
• fixed border looking weird when height/width is less than corner radius
• renamed bm_cairo_draw_rounded_path to bm_cairo_rounded_path because it was not actually drawing the path and only setting it as the shape in cairo.

I checked if the changes work with --fixed-height, --list, --line-height on x11 and wayland.

Also what is the reason that anti-aliasing is explicitly disabled in the code?

[Cloudef]

Not sure about the anti-alias, it was introduced in commit https://github.com/Cloudef/bemenu/commit/877ca82cc18feded7f9f4e8bc9661dbd506ada88