Automatically render image/ink annotations once annotations are synced or not exist in image cache

[mrtcode]

To display image/ink annotations in item pane, image rendering is also needed (which needs hidden browser and isn't fixed in fx102 branch yet).

Rendered images are also necessary if we want annotations in PDF reports.

It should reuse the same background processing logic as in #2906.

Related #2760.

Alternatively, we can just trigger annotation rendering on demand.