Fix compute_mbr for var-sized dimensions

[joe-maley]

There is a mistake in the logic of compute_mbr and compute_mbr_var that
assumes that the var-sized cells in the value tile do not span more than one
chunked buffer. If a var-sized cell contains 2 integer values, it is only true
that the bytes for an individual integer value do not span a chunked buffer.
The first integer value may be at the end of one chunked buffer and the second
integer may be on the next chunked buffer.

This could be easily fixed, but I also noticed that the write path (where these
functions are called) guarantees that tile chunked buffers will always be
contiguous. In this scenario, we can just revert both compute_mbr and
compute_mbr_var to the logic before the chunked buffer patch (commit hash
4b4b502).

[stavrospapadopoulos]

In the write path, for sure the values are (or should be) in a contiguous buffer.

[github-actions]

The backport to release-2.0.7 failed:

The process 'git' failed with exit code 1

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-2.0.7 release-2.0.7
# Navigate to the new working tree
cd .worktrees/backport-release-2.0.7
# Create a new branch
git switch --create backport-1732-to-release-2.0.7
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 9d7753249b7a1470e666e2520013f773322e9921
# Push it to GitHub
git push --set-upstream origin backport-1732-to-release-2.0.7
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-2.0.7

Then, create a pull request where the base branch is release-2.0.7 and the compare/head branch is backport-1732-to-release-2.0.7.