[core] restructure TileSource construction and callbacks

src/mbgl/tile/image_tile_source.cpp

@@ -0,0 +1,25 @@
+#include <mbgl/tile/image_tile_source.hpp>
+#include <mbgl/tile/raster_tile_data.hpp>
+#include <mbgl/storage/file_source.hpp>
+
+namespace mbgl {
+
+ImageTileSource::ImageTileSource(RasterTileData& tileData_,
+                                 const Resource& resource_,
+                                 FileSource& fileSource_)
+    : RasterTileSource(tileData_), resource(resource_), fileSource(fileSource_) {
+    request = fileSource.request(resource, [this](Response res) {
+        if (res.error) {
+            tileData.setData(std::make_exception_ptr(std::runtime_error(res.error->message)),
+                             nullptr, res.modified, res.expires);
+        } else if (res.notModified) {
+            return;
+        } else if (res.noContent) {
+            tileData.setData(nullptr, nullptr, res.modified, res.expires);
+        } else {
+            tileData.setData(nullptr, res.data, res.modified, res.expires);
+        }
+    });
+}
+
+} // namespace mbgl