Convert Bar to use river-shell-surface

I want to implement more functionality to the bar, similar to what
machi has in its bar, but it seems a lot easier to just handle the bar
with the rest of the manage/render loop that rwm and beansprout use.

To do that, I had to convert the bar to use river-shell-surface instead
of zwlr-layer-shell.

In that process, I also removed support for zwlr-layer-shell exclusive
zones. It made calculating the usable area for the layout more annoying.
If someone *really* wants, I would consider adding it back, but the only
thing I can think of that requires exclusive area is a bar, and we don't
really support other bars, so I don't think it's needed.

I also switched a couple of places to use saturating subtraction on
unsigned ints.

src/Window.zig

@@ -56,10 +56,7 @@ pub const PendingManage = struct {
 };
 
 pub const PendingRender = struct {
-    position: ?struct {
-        x: i32,
-        y: i32,
-    } = null,
+    position: ?struct { x: i32, y: i32 } = null,
 
     focused: ?bool = null,