pointer-constraints: implement protocol

Now with 50% less pointer warping!

The new implementation requires the user to move the cursor into the
constraint region before the constraint is activated in order to keep
behavior more predictable.

build.zig

@@ -108,6 +108,7 @@ pub fn build(b: *zbs.Builder) !void {
 
     scanner.generate("xdg_wm_base", 2);
     scanner.generate("zwp_pointer_gestures_v1", 3);
+    scanner.generate("zwp_pointer_constraints_v1", 1);
     scanner.generate("ext_session_lock_manager_v1", 1);
 
     scanner.generate("zriver_control_v1", 1);