Render Canvas to an arbitrarily-sized image

### What is the problem or limitation you are having?

I am building an app in which a preview of a document is displayed onscreen that is drawn using Canvas; however, I needed the ability to export that preview onto an arbitrarily-sized image.

### Describe the solution you'd like

A size parameter on as_image of Canvas that lets you export a scaled view of the Canvas.

This will be implemented in most backends by scaling the Canvas before rendering onto an image; reset_transform must be handled appropriately. Cocoa and iOS allows for setting a different logical size for an image than the physical size, which can be useful to implement this

### Describe alternatives you've considered

1. Status quo. One must manually resize the canvas, wait for layout, and then grab the image, and then reset image.
2. An canvas-like API on toga.Image that allows directly annotating an image.

### Additional context

A desktop implementation (with some additional fixes) without reset_transform support lives in johnzhou721/togax-frost, my personal component library. It has been used in johnzhou721/tiler successfully.

Start by locating Canvas.as_image in each backend and review the desktop implementation in johnzhou721/togax-frost, including its reset_transform limitation. Compare the Cocoa and iOS logical-versus-physical image sizing behavior. Done means a size parameter exports the canvas at the requested dimensions without manual resize and layout.

Written by the indexing model from the issue text.