Fix broken annotation icons in the reference tests

[timvandermeij]

Recently, most likely since Firefox Nightly got updated, we're seeing broken annotation icons: either the icon is gone or it's overlayed with text. The most recent run can be found in #18598. We should investigate what's causing this all of a sudden and if it's in fact a bug in PDF.js or a browser issue.

[timvandermeij]

I can't reproduce the issue by opening some of the affected PDFs in Firefox on Linux, which is an affected OS/browser pair according to the reference test output above, so it looks like the issue is limited to the reference testing framework itself rather than an issue in PDF.js itself. Most likely this points to an issue in the annotation rasterizer, which might be in need of changing anyway given that we've previously seen quite some intermittent failures in the reference tests where annotations suddenly weren't being rasterized/visible anymore...