Curved window borders

[MatthewScholefield]

I use lite xl on Wayland in GNOME (client side decorations) and I think it'd look amazing to have some subtle rounding of the edges on the window (when using client side window decorations). I think it should be feasible to do looking off the testshape.c example within the SDL repo, although I couldn't get the example working locally (I kept running into ERROR: Could not create shaped window for SDL_Shape.).

I may eventually try getting this working, but I just wanted to create the ticket to track. Feel free to close if there's no interest in including this even as a setting.

[Guldoman]

We're planning to revamp our CSD when we port to SDL3, which should allow to easily implement drawing them from the Lua side.

SDL3 implements a more flexible API than shaped windows, which allows to use transparent surfaces for windows.
This would even allow us to draw our own shadows.