Overview generation: fix multi-threaded bug, resulting in locks/crashes with GeoPackage in particular

[rouault]

Fixes #10245

[coveralls]

coverage: 69.246% (+1.3%) from 67.922%
when pulling 727a00d on rouault:fix_10245
into 5d4c2d3 on OSGeo:master.

[rouault]

The backport to release/3.9 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 366606f99f... Overview generation: fix multi-threaded bug, resulting in locks/crashes with GeoPackage in particular
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging gcore/gdal_priv.h
CONFLICT (content): Merge conflict in gcore/gdal_priv.h
Auto-merging gcore/overview.cpp
Auto-merging gcore/rasterio.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release/3.9 release/3.9
# Navigate to the new working tree
cd .worktrees/backport-release/3.9
# Create a new branch
git switch --create backport-10246-to-release/3.9
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 366606f99f15962a0dc17974e6bcf721834441be
# Push it to GitHub
git push --set-upstream origin backport-10246-to-release/3.9
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release/3.9

Then, create a pull request where the base branch is release/3.9 and the compare/head branch is backport-10246-to-release/3.9.

[rouault]

3.9 backport in #10255