Merge branch 'master' into enhanced_viewport_overlay

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

src/zoom_type.h

@@ -58,6 +58,7 @@ DECLARE_POSTFIX_INCREMENT(ZoomLevel)
 /** Type for storing the zoom level in a byte. */
 typedef SimpleTinyEnumT<ZoomLevel, byte> ZoomLevelByte;
 
+extern ZoomLevelByte _gui_zoom;
 #define ZOOM_LVL_GUI (_gui_zoom)
 
 #endif /* ZOOM_TYPE_H */