[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`.

test/api/custom_layer.test.cpp

@@ -3,7 +3,6 @@
 #include <mbgl/platform/gl_functions.hpp>
 #include <mbgl/map/map.hpp>
 #include <mbgl/map/map_options.hpp>
-#include <mbgl/util/default_thread_pool.hpp>
 #include <mbgl/gl/defines.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/storage/resource_options.hpp>
@@ -90,9 +89,8 @@ public:
 TEST(CustomLayer, Basic) {
     util::RunLoop loop;
 
-    ThreadPool threadPool(4);
-    HeadlessFrontend frontend { 1, threadPool };
-    Map map(frontend, MapObserver::nullObserver(), threadPool,
+    HeadlessFrontend frontend { 1 };
+    Map map(frontend, MapObserver::nullObserver(),
             MapOptions().withMapMode(MapMode::Static).withSize(frontend.getSize()),
             ResourceOptions().withCachePath(":memory:").withAssetPath("test/fixtures/api/assets"));
     map.getStyle().loadJSON(util::read_file("test/fixtures/api/water.json"));