Fix toggling visibility of group layers resets child layer mask blend modes

[nyalldawson]

And some other group layer related fixes

[nirvn]

Nice, very happy #53057 is fixed.

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply a25588c7423... Fix toggling visibility of group layers resets child layer
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".

stdout

[backport-54897-to-queued_ltr_backports 305db99360a] Invisible group layers should not be added to canvas
 Author: Nyall Dawson <nyall.dawson@gmail.com>
 Date: Tue Oct 10 14:43:50 2023 +1000
 1 file changed, 2 insertions(+), 1 deletion(-)
Auto-merging src/core/qgsgrouplayer.cpp
CONFLICT (content): Merge conflict in src/core/qgsgrouplayer.cpp
Auto-merging tests/src/python/test_qgsgrouplayer.py

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-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-54897-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick fef20dc04a2ea97092d7c08d427cc172c76f45e3,a25588c7423a7abd3e3e84205e4f7117f1efcb19,7b5ac045e342ca0b03a3f2d8d705576df32d78fb
# Push it to GitHub
git push --set-upstream origin backport-54897-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-54897-to-queued_ltr_backports.

[kannes]

Oh yessss, this made a colleague and me lose quite a bit of hair the other week. Thank you!