feat(mosaic): option to maintain aspect ratio of image

it would be great to get the aspect ratio of the image so you can set a width and it will use an appropriate height. Right now it goes a little crazy if both height AND width are not set.

The issue identifies the mosaic image-sizing behavior but names no file or test; start by locating the mosaic implementation and how it handles image width and height. The change is done when setting only a width derives an appropriate height from the image aspect ratio without the current sizing behavior becoming unstable.

Written by the indexing model from the issue text.