Option to show the camera bounds of the in-game view

### Is your feature request related to a problem? Please describe.

I'm making a little GBA game with tons of parallax. So I use the parallax feature of the Tiled editor to represent that. But I've noticed that the parallax appears far more extreme in the editor, because my view is likely zoomed out significantly, which scales the parallax scrolling.

So I thought it'd be nice to be able to see exactly what's going to appear in-game while editing, in the form of a little box in the center of the screen that has the same size as the in-game view.

This will likely benefit level designs that don't use parallax, as well, since level designers can more clearly see what's on-screen at any given moment.

### Describe the solution you'd like.

I suggest adding an option in, say, the map properties to specify the size of the in-game view (such as 240×160 pixels in my case). In addition, there should be a toggle in the "view" menu that simply draws a rectangle of that size in the middle of the viewport.

### Describe alternatives you've considered.

I've also come up with a simple workaround that I'm currently using, in which I have an object layer with a parallax factor of 0 (so it's fixed to the viewport) with a rectangle object in the center. This works pretty well, and the only real downsides are that it's a bit hacky and will require that I inform the engine that this isn't an actual object.

I'm honestly fine with it, so this proposal is really more about making something more standardized and less hacky than filling some gap in the editor's feature set.

EDIT: This hack also has the downside that the inside of the rectangle object is slightly tinted. It's not a lot, but it is slightly noticable.