fix a bunch of -Wall warnings

[benoit-pierre]

Rehash of #579, only the uncontroversial stuff, with amended commit messages.

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This change is 

[poire-z]

Pinging @NiLuJe for qimagescale.cpp quick review.

[poire-z]

CI suceeded on master after this is merged, even though part 2 was not yet merged.
I'm ok with that success :) but when looking at the output:
https://github.com/koreader/crengine/actions/runs/10140113071/job/28034656717
items with warnings do not standout vs. those without (no opening, no color). Is that expected ?
Meaning I'll open each of the item to look only twice a year - which is fine by me :)

[Frenzie]

It's probably intentional until that is merged so the warnings can be turned on without being distracting noise.

[benoit-pierre]

The thinking was more along the lines of: suppress spurious warnings, and rely on annotations. I need to check how multi-line matchers work, see if we can get all the details of a warning (e.g. the following reasoning notes by clang-tidy) in the annotation.

[benoit-pierre]

Note: there's also the summary in the workflow.