Image Layers: Allow using a sub-rect of the image

As the topic title states, it would be nice to be able to define a rectangle within an image and use that as the image layer. This would allow using atlases of background images instead of having each one as its own image, or using different sections of one image for different maps.

I’m currently doing this with custom properties on the Image Layer, but I can’t preview what I get within Tiled, and if I use a large atlas, it looks very silly and distracting while editing.

In this situation, you should create a tileset with your image if possible.
And create a tile layer with only one tile : the sub-rect of your image.
Like this, you will be able to preview the result directly on Tiled.

This would not allow me to mix image layers with regular-sized tiles, since Tiled doesn’t allow having different tile sizes per layer. And my background images aren’t necessarily a multiple of the usual tile size.
(I’d also prefer to avoid this for compatibility reasons, my engine doesn’t support multiple tilesets per map xP)