Fix cast_motion sometimes failing

- Fixes Godot physics failing when the cast Shape is inside of, or
already colliding with another Shape.
- Fixes Bullet physics failing when there is no motion.
- Ensures Godot and Bullet physics behave the same.
- Updates the documentation to exclude the caveats for the failures and
differences.

servers/physics_2d/space_2d_sw.cpp

@@ -269,10 +269,10 @@ bool Physics2DDirectSpaceStateSW::cast_motion(const RID &p_shape, const Transfor
 			continue;
 		}
 
-		//test initial overlap
+		//test initial overlap, ignore objects it's inside of.
 		if (CollisionSolver2DSW::solve(shape, p_xform, Vector2(), col_obj->get_shape(shape_idx), col_obj_xform, Vector2(), NULL, NULL, NULL, p_margin)) {
 
-			return false;
+			continue;
 		}
 
 		//just do kinematic solving