Add simple in-line implementation of rdcarray::push_back

* Coverity can't figure out that a pointer being push_back'd isn't
  leaking, possibly due to the call to insert.

renderdoc/api/replay/basic_types.h

@@ -158,7 +158,6 @@ public:
   bool empty() const { return usedCount == 0; }
   bool isEmpty() const { return usedCount == 0; }
   void clear() { resize(0); }
-  void push_back(const T &el) { insert(size(), &el, 1); }
   /////////////////////////////////////////////////////////////////
   // managing elements and memory
 
@@ -231,6 +230,15 @@ public:
     }
   }
 
+  void push_back(const T &el)
+  {
+    // in-line implementation here instead of insert()
+    const size_t lastIdx = size();
+    reserve(size() + 1);
+    new(elems + lastIdx) T(el);
+    setUsedCount(usedCount + 1);
+  }
+
   void insert(size_t offs, const T *el, size_t count)
   {
     const size_t oldSize = size();