[Feature Request]: save ashlar as separate layers

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

Hi!

Really love the ashlar pseudo-image format. However it creates an image that is not easily modifiable, i. e. I can't just open it in Krita/GIMP/Photoshop and move some images around, resize them without losing quality, etc.

Describe the solution you'd like

It would be awesome if the commands below produced ashlar images where all input images (and labels) are on separate layers. This could greatly improve the flexibility of this coder.

1. magick -label %f *.jpg 'ashlar:ashlar.psd'
   

2. magick -label %f *.jpg 'ashlar:ashlar.tiff[1920x1080+4+4]'
   

Describe alternatives you've considered

There could be a flag to make ashlar coder output positions of input images in the resulting collage so it can somehow be edited afterwards.

Some sort of machine-readable output for ashlar would also be really nice to make it easier to create collages of differently-sized videos with these steps:

1. Taking their first frames
2. Combine them with ashlar
3. Record their positions
4. Create a complex filtergraph to make a video where all input videos are nicely packed together

Additional context

Right now (v7.1.2-8) ashlar downscales images if canvas size is not specified. Try it with a single image:

magick input.jpg ashlar:collage.jpg

Here is input.jpg:

Here is collage.jpg (notice random border and lower quality):

Start by reproducing the two ashlar commands in the issue, including the single-image downscaling example. Then trace the ashlar coder and the PSD/TIFF output behavior to determine how input images and labels could remain separate layers. Done means the requested commands produce editable layered output without losing the source image quality.