Add layer shell support and fix floating windows

Now, I actually save the river-layer-shell-v1 and keep track of the
non-exclusive area. The layout calculation uses the usable area instead
of the entire output's geometry.

I removed boundary clamping for the floating windows because it was a
bit janky when hitting the edges. I'll probably add it back at some
point. I also made windows default to 75% of the usable area instead of
keeping their tiled size so that maximized windows look decent when
floating for the first time. Finally, since I removed the clamping, I
added a center_float keybind to center a floating window. If you're
cycling through focused windows and one isn't on the screen, you can use
the center_float bind to get the window visible again.

Replaced all divTrunc with divFloor to be consistent. I think they
should all be positive, anyways, so they'd be the same, but I like just
having one.

docs/CONFIGURATION.md

@@ -146,6 +146,7 @@ Full command reference:
 | `move_right`             | pixels           | Move floating window right                        |
 | `resize_width`           | pixels           | Resize floating window width (negative to shrink) |
 | `resize_height`          | pixels           | Resize floating window height (negative to shrink)|
+| `center_float`           |                  | Center the focused floating window on its output  |
 | `set_output_tags`        | tags (u32 bitmask) | Set the tags on the focused output              |
 | `set_window_tags`        | tags (u32 bitmask) | Set the tags on the focused window              |
 | `toggle_output_tags`     | tags (u32 bitmask) | Toggle a tag on the focused output              |

docs/TODO.md

@@ -4,6 +4,7 @@ These are in rough order of my priority, though no promises I do them in this or
 
 - [ ] Implement a river-tag-overlay clone
 - [ ] Implement an optional clock bar
+- [ ] Make a Rect struct to combine x, y, width, and height
 - [ ] Support window rules (float/tags/SSD by app-id/title)
 - [ ] Support overriding config location
 - [ ] Support configuring primary vs secondary stack side