[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`.

1 files changed, 2 insertions, 3 deletions

diff --git a/platform/default/include/mbgl/gl/headless_frontend.hpp b/platform/default/include/mbgl/gl/headless_frontend.hpp
index 18f0cfa537..d32c2e7768 100644
--- a/platform/default/include/mbgl/gl/headless_frontend.hpp
+++ b/platform/default/include/mbgl/gl/headless_frontend.hpp
@@ -12,7 +12,6 @@
 namespace mbgl {
 
 class FileSource;
-class Scheduler;
 class Renderer;
 class RendererBackend;
 class Map;
@@ -20,8 +19,8 @@ class TransformState;
 
 class HeadlessFrontend : public RendererFrontend {
 public:
-    HeadlessFrontend(float pixelRatio_, FileSource&, Scheduler&, const optional<std::string> programCacheDir = {}, GLContextMode mode = GLContextMode::Unique, const optional<std::string> localFontFamily = {});
-    HeadlessFrontend(Size, float pixelRatio_, FileSource&, Scheduler&, const optional<std::string> programCacheDir = {}, GLContextMode mode = GLContextMode::Unique, const optional<std::string> localFontFamily = {});
+    HeadlessFrontend(float pixelRatio_, FileSource&, const optional<std::string> programCacheDir = {}, GLContextMode mode = GLContextMode::Unique, const optional<std::string> localFontFamily = {});
+    HeadlessFrontend(Size, float pixelRatio_, FileSource&, const optional<std::string> programCacheDir = {}, GLContextMode mode = GLContextMode::Unique, const optional<std::string> localFontFamily = {});
     ~HeadlessFrontend() override;
 
     void reset() override;