Move systems to PostUpdate (#3)

I'm experiencing a little bit of jitter when having the camera track an
object as it moves. I believe it's because the object's transform is
updated, then the pixel camera plugin systems run, and then my camera
tracking system runs, causing there to be a one-frame delay before the
pixel camera's transform gets updated. This PR fixes it for me.

I believe it's correct to put these systems in `PostUpdate`, but I could
also make do with a `SystemSet` for these two if you prefer that.

src/lib.rs

@@ -16,7 +16,7 @@ impl Plugin for PixelCameraPlugin {
         use crate::systems::*;
 
         app.insert_resource(Msaa::Off).add_systems(
-            Update,
+            PostUpdate,
             (
                 init_camera,
                 update_viewport_size,