[core] Move setResourceTransform() to FileSource interface

include/mbgl/storage/file_source.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <mbgl/storage/resource_transform.hpp>
 #include <mbgl/storage/response.hpp>
 
 #include <mapbox/value.hpp>
@@ -80,6 +81,9 @@ public:
     virtual void setProperty(const std::string&, const mapbox::base::Value&){};
     virtual mapbox::base::Value getProperty(const std::string&) const { return {}; };
 
+    // When supported, sets the modifier of the requested resources.
+    virtual void setResourceTransform(ResourceTransform) {}
+
 protected:
     FileSource() = default;
 };

include/mbgl/storage/online_file_source.hpp

@@ -5,8 +5,6 @@
 
 namespace mbgl {
 
-class ResourceTransform;
-
 class OnlineFileSource : public FileSource {
 public:
     OnlineFileSource();
@@ -19,10 +17,7 @@ public:
     void resume() override;
     void setProperty(const std::string&, const mapbox::base::Value&) override;
     mapbox::base::Value getProperty(const std::string&) const override;
-
-    // OnlineFileSource interface.
-    // TODO: Would be nice to drop it to get uniform interface.
-    virtual void setResourceTransform(ResourceTransform);
+    void setResourceTransform(ResourceTransform) override;
 
 private:
     class Impl;