Ensure identify highlight works for very transparent symbols

[nyalldawson]

When mapping color for rendering highlighted (identified) features, don't allow pixels which were previously non-transparent to become totally transparent.

This can result in invisible highlights for very transparent symbols.

Fixes #54830

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply 1758684935... Cleanup and modernize QgsHighlight test
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

Auto-merging src/gui/qgshighlight.cpp
[backport-54864-to-queued_ltr_backports b842f2cf69] When mapping color for rendering highlighted (identified) features, don't allow pixels which were previously non-transparent to become totally transparent.
 Author: Nyall Dawson <nyall.dawson@gmail.com>
 Date: Fri Oct 6 13:41:05 2023 +1000
 1 file changed, 1 insertion(+), 1 deletion(-)
Auto-merging tests/src/python/test_qgshighlight.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgshighlight.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-54864-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 250c1560cf6a33fff62819cfde5d11e78af9598b,175868493532a0ce6a4d93cf5c661bfee97a8429,de3dafd0db3d17b126967fde4fa891de31081225,41def10ebbc7132b5090e1032c3cec225b0098e7
# Push it to GitHub
git push --set-upstream origin backport-54864-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-54864-to-queued_ltr_backports.