Fix operator[] for vector2d and vector3d being potentially UB (#15977)

We don't have a C++ expert on hand, but taking a pointer to one member
and expecting to access another by an offset is very fishy:
- for one, there could theoretically be padding
- the compiler might assume that we are only writing to that first member

The new code has shown to be free for constant parameter values.
Non-constant ones cause the assembly to have branches (why?), but we don't
use that much.

irr/src/OpenGL/Driver.cpp

@@ -104,8 +104,7 @@ static const VertexType &getVertexTypeDescription(E_VERTEX_TYPE type)
 	case EVT_TANGENTS:
 		return vtTangents;
 	default:
-		assert(false);
-		CODE_UNREACHABLE();
+		IRR_CODE_UNREACHABLE();
 	}
 }