Cache the result of GetImage() in Vehicle::UpdateViewport() where possible.

Sprite number is not cached if callback is made, or a variable access
outside a whitelist occurs.
Invalidate cached sprite number when direction or cargo changes,
or vehicle is marked dirty.

src/ship_cmd.cpp

@@ -259,6 +259,7 @@ Trackdir Ship::GetVehicleTrackdir() const
 void Ship::MarkDirty()
 {
 	this->colourmap = PAL_NONE;
+	this->cur_image_valid_dir = INVALID_DIR;
 	this->UpdateViewport(true, false);
 	this->UpdateCache();
 }