Subpixel positioning sometimes screws up hinting

(Originally reported downstream, but it seems to be a Cairo issue, so filing an issue here)

After an upgrade to Fedora 34, some font issues started to appear in Epiphany that we seem to have tracked down to Cairo's new subpixel positioning. When full hinting is enabled (using the old TTF engine), some glyphs (or words) get slightly misplaced so that the hinting is no longer able to do it's job.

Oddly enough it only seems to affect Epiphany this way as everything else correctly hints and positions things. No idea why.

After some digging I found one more glitch though. In GNOME software entire paragraphs can get misplaced, and it seems to happen vertically rather than horisontally.

I wonder if this and #390 has some overlap? The screenshot there shows not only cropping, but messed up hinting.

Downstream bug has screenshots. I can duplicate them here if desired.