[core] Make the BackgroundScheduler a singleton

- Do not carry it over everywhere as parameter, it is a shared
  instance anyway and the lifecycle is pretty much the app lifecycle
  from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
  do tasks in the background, we don't make assumptions if it is a
  thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.

bin/render.cpp

@@ -5,7 +5,6 @@
 #include <mbgl/util/default_styles.hpp>
 
 #include <mbgl/gl/headless_frontend.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/style/style.hpp>
 
 #include <args.hxx>
@@ -76,9 +75,8 @@ int main(int argc, char *argv[]) {
 
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
-    HeadlessFrontend frontend({ width, height }, pixelRatio, threadPool);
-    Map map(frontend, MapObserver::nullObserver(), threadPool,
+    HeadlessFrontend frontend({ width, height }, pixelRatio);
+    Map map(frontend, MapObserver::nullObserver(),
             MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()).withPixelRatio(pixelRatio),
             ResourceOptions().withCachePath(cache_file).withAssetPath(asset_root).withAccessToken(std::string(token)));