[core] separate Backend from View for headless rendering

platform/node/src/node_map.cpp

@@ -773,15 +773,16 @@ void NodeMap::QueryRenderedFeatures(const Nan::FunctionCallbackInfo<v8::Value>&
 }
 
 NodeMap::NodeMap(v8::Local<v8::Object> options) :
-    view(sharedDisplay(), [&] {
+    backend(sharedDisplay()),
+    view([&] {
         Nan::HandleScope scope;
         return Nan::Has(options, Nan::New("ratio").ToLocalChecked()).FromJust() ? Nan::Get(options, Nan::New("ratio").ToLocalChecked()).ToLocalChecked()->NumberValue() : 1.0;
     }()),
     threadpool(),
-    map(std::make_unique<mbgl::Map>(view, *this, threadpool, mbgl::MapMode::Still)),
+    map(std::make_unique<mbgl::Map>(backend, view, view.getPixelRatio(), *this, threadpool, mbgl::MapMode::Still)),
     async(new uv_async_t) {
 
-    view.setMapChangeCallback([&](mbgl::MapChange change) {
+    backend.setMapChangeCallback([&](mbgl::MapChange change) {
         if (change == mbgl::MapChangeDidFailLoadingMap) {
             throw std::runtime_error("Requires a map style to be a valid style JSON");
         }

platform/node/src/node_map.hpp

@@ -4,6 +4,7 @@
 
 #include <mbgl/map/map.hpp>
 #include <mbgl/storage/file_source.hpp>
+#include <mbgl/platform/default/headless_backend.hpp>
 #include <mbgl/platform/default/headless_view.hpp>
 
 #pragma GCC diagnostic push
@@ -53,6 +54,7 @@ public:
 
     std::unique_ptr<mbgl::AsyncRequest> request(const mbgl::Resource&, mbgl::FileSource::Callback);
 
+    mbgl::HeadlessBackend backend;
     mbgl::HeadlessView view;
     NodeThreadPool threadpool;
     std::unique_ptr<mbgl::Map> map;