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

src/mbgl/renderer/render_source.cpp

@@ -21,7 +21,7 @@ namespace mbgl {
 using namespace style;
 
 std::unique_ptr<RenderSource> RenderSource::create(const Immutable<Source::Impl>& impl,
-                                                   std::shared_ptr<Scheduler> threadPool_) {
+                                                   const TaggedScheduler& threadPool_) {
     switch (impl->type) {
         case SourceType::Vector:
             return std::make_unique<RenderVectorSource>(staticImmutableCast<VectorSource::Impl>(impl),