Point cloud editing: rewrite COPC when committing changes

[wonder-sk]

This PR extends work started in #59973 - it adds functionality to store changes to attributes of a point cloud to a COPC file.

There is a new class QgsCopcUpdate that:

1. reads and existing COPC file
2. writes a new COPC file with some chunks (nodes) with updated content

Writing of new COPC files is fast, because typically only a small amount of points are modified and so most of the chunks are copied as they were. There are also no changes to the hierarchy.

QgsPointCloudLayer::commitChanges() has been modified to use QgsCopcUpdate. A new file is created next to the original file. The original file is then renamed, and newly created file is renamed to effectively replace it. In the final step, the original file is removed.

[github-actions]

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[uclaros]

Nicely done!

Just some tidying up comments.

Also a question: We mostly pass QgsPointCloudNodeId by const ref and name it n, while here it is mostly passed by val and named k. Should we stick with one or the other? With QgsPointCloudNodeId being practically 4 ints it doesn't really matter, right?

[wonder-sk]

Thanks for the review!

We mostly pass QgsPointCloudNodeId by const ref and name it n, while here it is mostly passed by val and named k.

yeah I have changed it now to const ref for consistency, and also renamed k (as for voxel Key) to n ...