Codechange: Spell 'Viewport' consistently

Some places in the codebase misspell 'Viewport' as 'ViewPort' or 'view_port'.
This patch makes everything consistent.

src/vehicle.cpp

@@ -1162,7 +1162,7 @@ void ViewportAddVehicles(DrawPixelInfo *dpi)
  * @param y  Y coordinate in the viewport.
  * @return Closest vehicle, or \c nullptr if none found.
  */
-Vehicle *CheckClickOnVehicle(const ViewPort *vp, int x, int y)
+Vehicle *CheckClickOnVehicle(const Viewport *vp, int x, int y)
 {
 	Vehicle *found = nullptr;
 	uint dist, best_dist = UINT_MAX;