advance image request queue when request aborted

[ansis]

fixes #7637

Aborted requests never decremented the number of active requests because the main callback is never called. This fixes that by making getImage return a wrapped version of the getArrayBuffer return value that also advances the queue.

Within advanceImageRequestQueue we need to change that it hasn't already been advanced for that request because sometimes cancel() is called after the request has completed.

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• manually test the debug page

[ansis]

@mourner can I get a review of the new commit?