fix: webgl: selection of nodes with background-opacity: 0

[mikekucera]

Associated issues:

• WebGL: group selection in bounding boxes broken #3406

Notes re. the content of the pull request. Give context to reviewers or serve as a general record of the changes made. Add a screenshot or video to demonstrate your new feature, if possible.

• When the WebGL renderer is in PICKING mode, if a node passes the visibility check its background-opacity is ignored and always set to 1. This ensures the node is still clickable/selectable.

Note: I did not thoroughly test this. Its a good idea to do some extra testing before merging.

Checklist

Author:

• The proper base branch has been selected. New features go on unstable. Bug-fix patches can go on either unstable or master.
• Automated tests have been included in this pull request, if possible, for the new feature(s) or bug fix. Check this box if tests are not pragmatically possible (e.g. rendering features could include screenshots or videos instead of automated tests).
• The associated GitHub issues are included (above).
• Notes have been included (above).
• For new or updated API, the index.d.ts Typescript definition file has been appropriately updated.

Reviewers:

• All automated checks are passing (green check next to latest commit).
• At least one reviewer has signed off on the pull request.
• For bug fixes: Just after this pull request is merged, it should be applied to both the master branch and the unstable branch. Normally, this just requires cherry-picking the corresponding merge commit from master to unstable -- or vice versa.