Spoof picture-in-picture sizes under RFP

This is the actionable follow-up/changelog item for #44725 (closed).

This has been upstreamed already as 76ee4dd68e5dd071a47a3b2c72b522f4e1aa8124 (Bug 2063031), and in 5 days it hasn't been backed yet.

So, I suppose we can proceed with the backport. I'm considering to request an ESR-uplift request upstream as well.