Indicate fallthrough to prevent a GCC warning

Without this here, GCC 11 and 12 will print a warning during compilation about a possible fallthrough, but we're doing that on purpose, so this simply adds an indicator to the compiler that we know what we're doing.

src/ActorMultiVertex.cpp

@@ -502,6 +502,7 @@ void ActorMultiVertex::UpdateAnimationState(bool force_update)
 					break;
 				}
 			}
+        [[fallthrough]];
 		case DrawMode_QuadStrip:
 			for (std::size_t i = first; i < last; ++i)
 			{