Fix incorrect frame duration in last frame in animation mode

[nyalldawson]

The final frame must always have the same duration as other frames,
even if this means we go past the end of the animation range. Also
avoid showing a "zero length" frame at the end of the animation
if the frame duration fits exactly into the overall animation
range.

Fixes #40777

[nirvn]

The additional test logic looks sound to me.

[qgis-bot]

The backport to queued_ltr_backports failed:

The process '/usr/bin/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-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-43660-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 0221122ceb18265d6596716b74a60196f6d91389,fb90f1b09e6fa48f6896cd53b65544efc253e280,d0328c25aa765234248a6dc53bc30f9de56b5b18
# Push it to GitHub
git push --set-upstream origin backport-43660-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-43660-to-queued_ltr_backports.