Add TaggedScheduler, couple lifetime of tasks and orchestrator (#2398)

platform/glfw/glfw_renderer_frontend.cpp

@@ -2,6 +2,7 @@
 
 #include <mbgl/renderer/renderer.hpp>
 #include <mbgl/gfx/backend_scope.hpp>
+#include <mbgl/gfx/renderer_backend.hpp>
 #include <mbgl/util/instrumentation.hpp>
 
 GLFWRendererFrontend::GLFWRendererFrontend(std::unique_ptr<mbgl::Renderer> renderer_, GLFWView& glfwView_)
@@ -27,6 +28,10 @@ void GLFWRendererFrontend::update(std::shared_ptr<mbgl::UpdateParameters> params
     glfwView.invalidate();
 }
 
+const mbgl::TaggedScheduler& GLFWRendererFrontend::getThreadPool() const {
+    return glfwView.getRendererBackend().getThreadPool();
+}
+
 void GLFWRendererFrontend::render() {
     MLN_TRACE_FUNC();
 

platform/glfw/glfw_renderer_frontend.hpp

@@ -18,6 +18,7 @@ public:
     void setObserver(mbgl::RendererObserver&) override;
 
     void update(std::shared_ptr<mbgl::UpdateParameters>) override;
+    const mbgl::TaggedScheduler& getThreadPool() const override;
     void render();
 
     mbgl::Renderer* getRenderer();