Implement wallpaper rendering with multi-output support

This actually renders a wallpaper for each output using the newly added
Buffer and BufferPool for shared-memory surfaces and creates a
wlr-layer-shell surface per output. Right now, each wallpaper
shares the same wallpaper (though scaled to each).

wl_output globals get added to a HashMap that is used by Output when it
gets an output event.

Fix null-safety in WindowManager when no seats/outputs exist and route
Window dimensions through pending_manage.

build.zig

@@ -111,6 +111,6 @@ pub fn build(b: *std.Build) void {
     exe_check.linkSystemLibrary("pixman-1");
     exe_check.linkSystemLibrary("xkbcommon");
 
-    const check = b.step("check", "Check if beanbag compiles");
+    const check = b.step("check", "Check if beansprout compiles");
     check.dependOn(&exe_check.step);
 }