Move most backend functionality to VideoCommon

Source/Core/VideoBackends/OGL/StreamBuffer.h

@@ -19,6 +19,8 @@ public:
   static std::unique_ptr<StreamBuffer> Create(u32 type, u32 size);
   virtual ~StreamBuffer();
 
+  u32 GetGLBufferId() const { return m_buffer; }
+  u32 GetSize() const { return m_size; }
   u32 GetCurrentOffset() const { return m_iterator; }
 
   /* This mapping function will return a pair of:
@@ -64,4 +66,4 @@ private:
 
   std::array<GLsync, SYNC_POINTS> m_fences{};
 };
-}
+}  // namespace OGL