OGL/VertexManager: Make vertex and index buffer handles private

These are only ever read, but not written to outside of the VertexManager class.

Source/Core/VideoBackends/OGL/Render.cpp

@@ -1796,7 +1796,7 @@ void Renderer::RestoreAPIState()
 
   ProgramShaderCache::BindLastVertexFormat();
   const VertexManager* const vm = static_cast<VertexManager*>(g_vertex_manager.get());
-  glBindBuffer(GL_ARRAY_BUFFER, vm->m_vertex_buffers);
+  glBindBuffer(GL_ARRAY_BUFFER, vm->GetVertexBufferHandle());
 
   OGLTexture::SetStage();
 }