render implement thread, wrap GL commands into tasks

src/canvas_modes.cpp

@@ -213,6 +213,7 @@ void CanvasModePen::on_MouseEvent(MouseEvent* me, glm::vec2& loc)
 
 void CanvasModePen::on_Draw(const glm::mat4& ortho, const glm::mat4& proj, const glm::mat4& camera)
 {
+    assert(App::I.is_render_thread());
     if (m_draw_tip)
     {
         const auto& brush = Canvas::I->m_current_brush;