Finish migration to qsizetype [QtWebEngine]

Let's be honest: the change from int to qsizetype sizes in Qt 6.0 was more than rushed. Half the API and a good chunk of implementation code is still using int when it should be using qsizetype, and we have been papering over this issue by making qMin() allow mixed-type arguments and, apparently, by removing the annoying MSVC warnings that, in Qt 5, so nicely pin-pointed all these implicit conversions as hard errors.

There were several real bugs in e.g. the QBuffer code corresponding to int/qsizetype, so there's probably more.

Acceptance criterium: every int has been inspected and, if needed, ported to qsizetype. Unfortunately, relying on compiler warnings isn't enough, as explicit casts won't raise warnings, but need to be ported nonetheless.