[core] make padding optional when camera position is requested.

When padding is not provided, the current one that's cached in the TransformState is going to be returned.

src/mbgl/map/transform.cpp

@@ -63,7 +63,7 @@ void Transform::resize(const Size size) {
 
 #pragma mark - Camera
 
-CameraOptions Transform::getCameraOptions(const EdgeInsets& padding) const {
+CameraOptions Transform::getCameraOptions(optional<EdgeInsets> padding) const {
     return state.getCameraOptions(padding);
 }