Make Yoga thread safe

[nokia6686]

Issue ref: #769

Problem

• gNodeInstanceCount, gConfigInstanceCount, gCurrentGenerationCount, gDepth are global variables in Yoga.cpp. They are not safe when using multithreading.

My idea

• Uses atomic for gNodeInstanceCount and gConfigInstanceCount because of counting instances only.
• Store gCurrentGenerationCount and gDepth in root node for multithreading.

My solution

• Uses for gNodeInstanceCount and gConfigInstanceCount

std::atomic<int32_t> gNodeInstanceCount(0);
std::atomic<int32_t> gConfigInstanceCount(0);

• Every Yoga Node has its root reference (root-ref). New node's root point to itself. Root-ref will be updated when inserting children.
• gCurrentGenerationCount and gDepth are stored in YGNode.
• When updating children, we must update root-ref for each new child.

struct YGNode {
...
  // We must setChildRoot() for new child
  void replaceChild(YGNodeRef oldChild, YGNodeRef newChild);
  void replaceChild(YGNodeRef child, uint32_t index);
  void insertChild(YGNodeRef child, uint32_t index);
...
  private:
    YGNodeRef pRoot = nullptr;

  public:
    uint32_t  gCurrentGenerationCount = 0;
    uint32_t  gDepth = 0;
    YGNodeRef getRoot();
    void setChildRoot(YGNodeRef node);
}
...
void YGNode::setChildRoot(YGNodeRef node) {
  node->pRoot = getRoot();
  for (auto child: node->children_) {
    setChildRoot(child);
  }
}

• Global variables are accessed via root-ref

//gCurrentGenerationCount++;
node->getRoot()->gCurrentGenerationCount++;

//gDepth++;
node->getRoot()->gDepth++;

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