Bug 948128. Add StackArray helper. r=Bas

This is helpful for when we want to convert between
different types and we need a short lived piece of memory.

--HG--
extra : rebase_source : b841a535cb1ddd837df4b34949bd34054c438c67

gfx/2d/StackArray.h

@@ -0,0 +1,30 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A handy class that will allocate data for size*T objects on the stack and
+ * otherwise allocate them on the heap. It is similar in purpose to nsAutoTArray */
+
+template <class T, size_t size>
+class StackArray
+{
+public:
+  StackArray(size_t count) {
+    if (count > size) {
+      mData = new T[count];
+    } else {
+      mData = mStackData;
+    }
+  }
+  ~StackArray() {
+    if (mData != mStackData) {
+      delete[] mData;
+    }
+  }
+  T& operator[](size_t n) { return mData[n]; }
+  const T& operator[](size_t n) const { return mData[n]; }
+  T* data() { return mData; };
+private:
+  T mStackData[size];
+  T* mData;
+};