[mvt] implement step expressions + improve conversion of sprites

[3nids]

this implements step expressions

it also fixes some cases where the sprite was not defined (mainly when data defined) + support step definition

see https://docs.mapbox.com/style-spec/reference/expressions/#step

fixes #57989

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 0f762cc)

[3nids]

This is ready to merge.

[nirvn]

@3nids , glad this is being addressed -- only one doc comment.

[nirvn]

Thanks for the optimization.

[nirvn]

Do we skip \since when it's a private API even though it's exposed to python?

[qgis-bot]

The backport to release-3_38 failed:

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

stderr

error: could not apply b66953a0c1a... handle zoom in parse expression
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".
hint: Disable this message with "git config advice.mergeConflict false"

stdout

Auto-merging src/core/vectortile/qgsmapboxglstyleconverter.cpp
Auto-merging src/core/vectortile/qgsmapboxglstyleconverter.h
Auto-merging tests/src/python/test_qgsmapboxglconverter.py
[backport-58233-to-release-3_38 832d1037ebc] [mvt] improve conversion of sprites
 Author: Denis Rouzaud <denis.rouzaud@gmail.com>
 Date: Wed Jul 24 12:04:23 2024 +0200
 7 files changed, 198 insertions(+), 44 deletions(-)
 create mode 100644 tests/testdata/vector_tile/sprites/swisstopo-sprite@2x.json
 create mode 100644 tests/testdata/vector_tile/sprites/swisstopo-sprite@2x.png
Auto-merging src/core/vectortile/qgsmapboxglstyleconverter.cpp
Auto-merging src/core/vectortile/qgsmapboxglstyleconverter.h
Auto-merging tests/src/python/test_qgsmapboxglconverter.py
[backport-58233-to-release-3_38 506cdb8b6ad] implement step expressions
 Author: Denis Rouzaud <denis.rouzaud@gmail.com>
 Date: Wed Jul 24 18:33:11 2024 +0200
 5 files changed, 129 insertions(+), 11 deletions(-)
Auto-merging src/core/vectortile/qgsmapboxglstyleconverter.cpp
Auto-merging tests/src/python/test_qgsmapboxglconverter.py
[backport-58233-to-release-3_38 6cec5d3eaae] fix some tests
 Author: Denis Rouzaud <denis.rouzaud@gmail.com>
 Date: Wed Jul 24 20:43:13 2024 +0200
 2 files changed, 16 insertions(+), 13 deletions(-)
Auto-merging src/core/vectortile/qgsmapboxglstyleconverter.cpp
CONFLICT (content): Merge conflict in src/core/vectortile/qgsmapboxglstyleconverter.cpp

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-3_38 release-3_38
# Navigate to the new working tree
cd .worktrees/backport-release-3_38
# Create a new branch
git switch --create backport-58233-to-release-3_38
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 0d33487affed2fb952c6ab88bf5bb8b53a1e99cc,9ddd2de832668a5a5e25719bcc70f07f0b1da507,6713b22a9655a73aa32821a2e4513dc1fbccefb7,b66953a0c1aeeab1eaf356a90c4ade8c29f6547e,79e3e48fb9da15d3c251c82b3394d191030b0aca,c93463f91287cea501a3c4f27fbb4abdb6e44e09,3931301c4a87aac5f08579458e7be9a4391ef789,b602cb70fe58f9525539abc37fecbb7a9be0cdae,0fd10899de123717bce75017e8bf3db761d80ee1,165672d823ad52c3c28964a7e9eeb30b2e786f5a,0f762cce15a552ead94ee3ddddcceb54c114daf2
# Push it to GitHub
git push --set-upstream origin backport-58233-to-release-3_38
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_38

Then, create a pull request where the base branch is release-3_38 and the compare/head branch is backport-58233-to-release-3_38.