Freehand annotations in image occlusion

Associated forum thread: https://forums.ankiweb.net/t/urgent-feature-request-drawing-and-highlighting-in-image-occlusion-mode/40309

It requests the ability to add annotations to images, such as:

- freehand drawing of arrows & text
- highlighting portions of the image

The latter could perhaps be done more efficiently with semi-transparent rectangles, but the former will probably require serializing the canvas to a pixel-based image, perhaps using the same dimensions as the original image, and overlaid above the original image.

A semi-related issue that may make sense to consider at the same time: we have [another user](https://forums.ankiweb.net/t/labels-for-image-occlusion/40181) wanting to use various shapes as permanent additions (like the text tool), and wanting to add text labels as occlusions.

Another semi-related issue is that currently the cloze deletion # for shapes is lost when they are edited: #2975

@krmanik @glutanimate and others, do you have any thoughts on how these features should fit together / be implemented?

Start with the linked forum thread and issue #2975 to understand the requested annotation and shape-editing behavior. No implementation files or tests are named, and the scope still needs design decisions about drawing, highlighting, labels, and how these interact with occlusions. Done would require an agreed design and complete support for the selected annotation features.

Written by the indexing model from the issue text.