Notes on conflict resolution:
* MarkTileDirtyByTile gained an extra param on both sides of the merge
Move bridge level offset to be after zoom level param, as it's used less.
* Add zoom level params to MarkBridgeDirty functions
* Fix undefined behaviour in colour_index cycling in ViewportMapDraw
Conflicts:
src/clear_cmd.cpp
src/pbs.cpp
src/rail_cmd.cpp
src/toolbar_gui.cpp
src/train_cmd.cpp
src/vehicle.cpp
src/viewport.cpp
src/viewport_func.h
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