Introduce CSSNodeCopyStyle

Summary:
- Add a convenience function to copy all style from one node to another.
- CSSNodeIsDirty will return true after this function if any style was changed in the destination node.

Reviewed By: emilsjolander

Differential Revision: D4188514

fbshipit-source-id: af8d7fb5e8688c64aea05ce7ad23fff9233fb441

CSSLayout/CSSLayout.c

@@ -341,6 +341,13 @@ bool CSSNodeIsDirty(const CSSNodeRef node) {
   return node->isDirty;
 }
 
+void CSSNodeCopyStyle(const CSSNodeRef dstNode, const CSSNodeRef srcNode) {
+  if (memcmp(&dstNode->style, &srcNode->style, sizeof(CSSStyle)) != 0) {
+    memcpy(&dstNode->style, &srcNode->style, sizeof(CSSStyle));
+    _CSSNodeMarkDirty(dstNode);
+  }
+}
+
 inline float CSSNodeStyleGetFlexGrow(CSSNodeRef node) {
   if (!CSSValueIsUndefined(node->style.flexGrow)) {
     return node->style.flexGrow;