[core] Eliminate duplicate Necessity types

src/mbgl/tile/raster_tile.cpp

@@ -60,7 +60,7 @@ Bucket* RasterTile::getBucket(const style::Layer&) {
 }
 
 void RasterTile::setNecessity(Necessity necessity) {
-    loader.setNecessity(static_cast<TileLoader<RasterTile>::Necessity>(necessity));
+    loader.setNecessity(necessity);
 }
 
 void RasterTile::cancel() {

src/mbgl/tile/tile.hpp

@@ -8,6 +8,7 @@
 #include <mbgl/renderer/bucket.hpp>
 #include <mbgl/text/placement_config.hpp>
 #include <mbgl/tile/geometry_tile_data.hpp>
+#include <mbgl/storage/resource.hpp>
 
 #include <string>
 #include <memory>
@@ -32,10 +33,12 @@ public:
 
     void setObserver(TileObserver* observer);
 
-    enum class Necessity : bool {
-        Optional = false,
-        Required = true,
-    };
+    // Tiles can have two states: optional or required.
+    // - optional means that only low-cost actions should be taken to obtain the data
+    //   (e.g. load from cache, but accept stale data)
+    // - required means that every effort should be taken to obtain the data (e.g. load
+    //   from internet and keep the data fresh if it expires)
+    using Necessity = Resource::Necessity;
 
     virtual void setNecessity(Necessity) = 0;
 

src/mbgl/tile/tile_loader.hpp

@@ -2,6 +2,7 @@
 
 #include <mbgl/util/noncopyable.hpp>
 #include <mbgl/storage/resource.hpp>
+#include <mbgl/tile/tile.hpp>
 
 namespace mbgl {
 
@@ -17,21 +18,13 @@ class UpdateParameters;
 template <typename T>
 class TileLoader : private util::noncopyable {
 public:
-    // TileSources can have two states: optional or required.
-    // - optional means that only low-cost actions should be taken to obtain the data
-    //   (e.g. load from cache, but accept stale data)
-    // - required means that every effort should be taken to obtain the data (e.g. load
-    //   from internet and keep the data fresh if it expires)
-    enum class Necessity : bool {
-        Optional = false,
-        Required = true,
-    };
-
     TileLoader(T&,
                const OverscaledTileID&,
                const style::UpdateParameters&,
                const Tileset&);
 
+    using Necessity = Resource::Necessity;
+
     void setNecessity(Necessity newNecessity) {
         if (newNecessity != necessity) {
             necessity = newNecessity;

src/mbgl/tile/vector_tile.cpp

@@ -77,7 +77,7 @@ VectorTile::VectorTile(const OverscaledTileID& id_,
 }
 
 void VectorTile::setNecessity(Necessity necessity) {
-    loader.setNecessity(static_cast<TileLoader<VectorTile>::Necessity>(necessity));
+    loader.setNecessity(necessity);
 }
 
 void VectorTile::setData(std::shared_ptr<const std::string> data_,