BMFF Performance boost: don't read boxes we're not interested in

[dhoulder]

This speeds up handling of large ISOBMFF files by not reading the mdat box.
Fixes #1961

Before:

david@blackbox:build$ time bash -c 'for i in $(seq 1 100); do ./bin/exiv2 -pp ~/Downloads/Canon-R5.CR3 >/dev/null; done'

real	0m2.937s
user	0m0.782s
sys	0m2.159s

After:

david@blackbox:build$ time bash -c 'for i in $(seq 1 100); do ./bin/exiv2 -pp ~/Downloads/Canon-R5.CR3 >/dev/null; done'

real	0m0.534s
user	0m0.327s
sys	0m0.214s

[dhoulder]

@Mergifyio backport 0.27-maintenance

[mergify]

backport 0.27-maintenance

🟠 Waiting for conditions

• merged [:pushpin: backport requirement]

[kmilos]

LGTM 👍

Can you please do a tiny cosmetic whitespace change on L78 please while we're committing to this file? In the last PR (CR3 preview) I added a space to the four-byte sig of "xml ", but that then pushed the comment to the right... can you just re-align the comment please?

[1div0]

Nice, LG2M.

[kevinbackhouse]

The macOS build failures look like a new problem related to conan. They're not caused by this PR.

[mergify]

backport 0.27-maintenance

✅ Backports have been created

• #1975 Performance boost: don't read boxes we're not interested in (backport #1974) has been created for branch 0.27-maintenance