Fix position on root node with RTL direction

Summary:
If the root node has a position and we have a RTL layout, that position must be like LTR direction. See #477.
Closes facebook/yoga#502

Differential Revision: D4867144

Pulled By: emilsjolander

fbshipit-source-id: b5ad3d87e7054090da12d7665a3d1abe8496a548

ReactCommon/yoga/yoga/Yoga.c

@@ -1324,8 +1324,12 @@ static void YGNodeSetPosition(const YGNodeRef node,
                               const float mainSize,
                               const float crossSize,
                               const float parentWidth) {
-  const YGFlexDirection mainAxis = YGResolveFlexDirection(node->style.flexDirection, direction);
-  const YGFlexDirection crossAxis = YGFlexDirectionCross(mainAxis, direction);
+  /* Root nodes should be always layouted as LTR, so we don't return negative values. */
+  const YGDirection directionRespectingRoot = node->parent != NULL ? direction : YGDirectionLTR;
+  const YGFlexDirection mainAxis =
+      YGResolveFlexDirection(node->style.flexDirection, directionRespectingRoot);
+  const YGFlexDirection crossAxis = YGFlexDirectionCross(mainAxis, directionRespectingRoot);
+
   const float relativePositionMain = YGNodeRelativePosition(node, mainAxis, mainSize);
   const float relativePositionCross = YGNodeRelativePosition(node, crossAxis, crossSize);