Support for rounding windows and server-side shadows

[Drakulix]

Can finally be tested, but also needs accompanying cosmic-settings work to expose the new experimental options.

[tobyrg]

I've noticed that the left and right sides of a window there is a small gap between window and the active hint. Seems to have been introduced with these changes as I don't see it on the latest commit on master.

[git-f0x]

The shadows look great!
Some issues regarding clipping I noticed:
When all the options in Settings are turned off, SSD windows end up with weird borders (the rounding is on opposite sides than it should be):

Clipping for the bottom corners of SSD apps doesn't work:

Clipping for floating non-libcosmic CSD apps makes them impossible to resize by grabbing an edge.
Edit: Qt apps seems to not be affected as much (potentially because of resize border placement?), since the grab handle appears and they can be resized. But GTK and others (e.g. Zed) are affected.

[TheESN]

Do I understand correctly that rounding window edges here affect not only apps with SSDs, but with CSDs too?