Implement window shadows

[YaLTeR]

Closes #927. Shadows are disabled by default and can be enabled on the layout {} or in a window rule.

@timgott want to take a glance just in case? I more or less followed your advice.

Everyone else: please give it a try. I'm looking for feedback:

• any graphical glitches?
• any performance issues?
• do the options and option names make sense? Anything that could be named better?
• do the defaults make sense?

I checked what options and defaults other implementations have (wayfire-shadows, swayfx, hyprland, fht) and arrived at the system in this PR. width, spread, offset and color mean the same thing as in CSS box-shadow. The default width of 30 is slightly lower than in wayfire-shadows and swayfx (44 and 40 respectively, converted to niri units), but we have a slight default spread of 5.

I set draw-behind-window to true by default because otherwise CSD rounded corners will look broken. However, with draw-behind-window false, niri will fully respect its own rounded corners (geometry-corner-radius) with no artifacts.

There's no "only floating" setting because you can window-rule it.

Contrary to other implementations, I decided to slightly reduce the shadow opacity for inactive windows. This is how CSD shadows usually work and I think it makes sense.

Niri shadows render below the border and focus ring, in contrast to CSD shadows that render above the border and focus ring.