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/Context.zig

@@ -216,12 +216,6 @@ pub fn manage(context: *Context) void {
                         log.err("Failed to create bar: {}", .{e});
                         continue;
                     };
-                    // If the output already has a wl_output, init the surface immediately
-                    if (output.wl_output != null) {
-                        output.bar.?.initSurface() catch |e| {
-                            log.err("Failed to init bar surface: {}", .{e});
-                        };
-                    }
                 }
             }
         }