POC: Add splitter support for vector data in leaflet

[zoran995]

What this PR does

Fixes #

Adds splitter support for points and billboards in Leaflet (2D mode). #7673 adds support for the splitter only in Cesium and this PR extends this support to Leaflet.

Approach: Each LeafletGeomVisualizer creates a dedicated Leaflet pane. All its layers (markers, circle markers, polylines, polygons) render into that pane. When splitting is active, CSS clip-path: polygon() is applied to the pane element — the same technique used for imagery tile layers.

Changes:

• Extracted shared getClipsForSplitter utility from duplicated code in ImageryProviderLeafletTileLayer and ImageryProviderLeafletGridLayer
• Modernised from deprecated CSS clip: rect() to clip-path: polygon() across all splitter consumers
• Added per-visualizer custom panes with @observable splitDirection / splitPosition and MobX autorun to LeafletGeomVisualizer
• Extended Leaflet._reactToSplitterChanges() to propagate split state to data source visualizers

Test me

1. Add a CSV or GeoJSON point dataset
2. Switch to 2D (Leaflet) mode
3. Enable the splitter
4. Set the dataset to LEFT or RIGHT split direction
5. Verify points/billboards clip at the split line
6. Drag the splitter — verify clipping updates in real time
7. Verify imagery layer splitting still works (no regressions)

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.