The journey map grouped its photo thumbnails and its plan and stay marks
on a grid laid over container pixels. Every pan shifted that grid against
the map, so photos near a cell edge changed groups after each move: the
count badges jumped and the thumbnail switched to another photo, although
the zoom never changed.
Both renderers now bucket in world pixels at the current zoom, Leaflet
through its own CRS (so GCJ-02 basemaps stay right) and GL through a
plain Web Mercator projection that ignores centre, bearing and pitch.
Panning keeps every group as it is; only a zoom regroups.
Closes#2889
Maurice
55b11f792a