Use main buffers for utility draws

Source/Core/VideoBackends/OGL/StreamBuffer.h

@@ -19,6 +19,8 @@ public:
   static std::unique_ptr<StreamBuffer> Create(u32 type, u32 size);
   virtual ~StreamBuffer();
 
+  u32 GetCurrentOffset() const { return m_iterator; }
+
   /* This mapping function will return a pair of:
    * - the pointer to the mapped buffer
    * - the offset into the real GPU buffer (always multiple of stride)