Fix PDF export size of elevation plots

[nyalldawson]

QPdfWriter doesn't return the output resolution as the device physicalDpiX(?!), so let's hack around this by just setting the output resolution to match the value reported by physicalDpiX...

Fixes #57880

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit aa8c8af)

[nyalldawson]

@troopa81 @m-kuhn @alexbruy @rouault @3nids can i get a review of this one? I'd love to get this fix in before tomorrow's set of releases...

[nirvn]

(For the record, that's this value: https://github.com/qt/qtbase/blob/a984d98c971234c111dd144681402c1e20e9d806/src/gui/painting/qpdf.cpp#L1413-L1415)

[qgis-bot]

The backport to release-3_34 failed:

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

stderr

error: could not apply aa8c8af4c67... Fix PDF export size of elevation plots
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 src/app/elevation/qgselevationprofilewidget.cpp
CONFLICT (content): Merge conflict in src/app/elevation/qgselevationprofilewidget.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_34 release-3_34
# Navigate to the new working tree
cd .worktrees/backport-release-3_34
# Create a new branch
git switch --create backport-58033-to-release-3_34
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick aa8c8af4c678de9dbf2024180f5b353c63b145ec
# Push it to GitHub
git push --set-upstream origin backport-58033-to-release-3_34
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_34

Then, create a pull request where the base branch is release-3_34 and the compare/head branch is backport-58033-to-release-3_34.