Fix artifacts when rendering filled line symbol

[nyalldawson]

Use geos to calculate the buffered line instead of Qt's path stroker,
as the later results in artifacts when line segments pass close
to other segments

Fixes #59689

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit f0110d6)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit f0110d6)

[nirvn]

Thanks I've been hit with this one before.

[qgis-bot]

The backport to release-3_40 failed:

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

stderr

error: could not apply ecf57ef9846... Add methods to directly buffer/offset GEOS geometry in QgsGeos
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 set advice.mergeConflict false"

stdout

Auto-merging src/core/symbology/qgssymbollayerutils.cpp
[backport-60554-to-release-3_40 609c68b4a45] Add method to directly convert QgsAbstractGeometry to QPolygonF lists
 Author: Nyall Dawson <[email protected]>
 Date: Wed Feb 12 13:14:14 2025 +1000
 4 files changed, 40 insertions(+), 5 deletions(-)
Auto-merging src/core/painting/qgsgeometrypaintdevice.cpp
Auto-merging src/core/symbology/qgssymbollayerutils.cpp
[backport-60554-to-release-3_40 ca06d8e8a6a] Move some useful utility methods to QgsSymbolLayerUtils
 Author: Nyall Dawson <[email protected]>
 Date: Wed Feb 12 13:22:47 2025 +1000
 8 files changed, 85 insertions(+), 40 deletions(-)
CONFLICT (modify/delete): python/PyQt6/core/auto_generated/geometry/qgsgeos.sip.in deleted in HEAD and modified in ecf57ef9846 (Add methods to directly buffer/offset GEOS geometry in QgsGeos).  Version ecf57ef9846 (Add methods to directly buffer/offset GEOS geometry in QgsGeos) of python/PyQt6/core/auto_generated/geometry/qgsgeos.sip.in left in tree.
CONFLICT (modify/delete): python/core/auto_generated/geometry/qgsgeos.sip.in deleted in HEAD and modified in ecf57ef9846 (Add methods to directly buffer/offset GEOS geometry in QgsGeos).  Version ecf57ef9846 (Add methods to directly buffer/offset GEOS geometry in QgsGeos) of python/core/auto_generated/geometry/qgsgeos.sip.in left in tree.
Auto-merging src/core/geometry/qgsgeos.cpp
Auto-merging src/core/geometry/qgsgeos.h

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_40 release-3_40
# Navigate to the new working tree
cd .worktrees/backport-release-3_40
# Create a new branch
git switch --create backport-60554-to-release-3_40
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 97f6dc5bbe9ab74120c33f3ab4b67958604d7408,649dc29c3213a10a2e402e83386ce8fa2dc1cf76,ecf57ef9846d45278d4c4c24fbd4e0dd93ed9b6a,f0110d6d0f1ef77f450ddc5210f6f8e71ac808f5
# Push it to GitHub
git push --set-upstream origin backport-60554-to-release-3_40
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_40

Then, create a pull request where the base branch is release-3_40 and the compare/head branch is backport-60554-to-release-3_40.