Introduce the ResourceLoader

ResourceLoader will handle all the tile changes and notifications
through the Source object owned by the Style. This object lives
entirely on the Map thread and should be only accessed by it.

The observer will receive notifications every time the tile data changes
and this will probably trigger a new rendering operation.

This first patch will delegate Source loading to the ResourceLoader.

src/mbgl/map/map_context.cpp

@@ -62,6 +62,7 @@ MapContext::~MapContext() {
 
     // Explicit resets currently necessary because these abandon resources that need to be
     // cleaned up by env.performCleanup();
+    resourceLoader.reset();
     style.reset();
     sprite.reset();
     painter.reset();
@@ -141,6 +142,7 @@ util::ptr<Sprite> MapContext::getSprite() {
 void MapContext::loadStyleJSON(const std::string& json, const std::string& base) {
     assert(Environment::currentlyOn(ThreadType::Map));
 
+    resourceLoader.reset();
     sprite.reset();
     style.reset();
 
@@ -153,12 +155,10 @@ void MapContext::loadStyleJSON(const std::string& json, const std::string& base)
     const std::string glyphURL = util::mapbox::normalizeGlyphsURL(style->glyph_url, data.getAccessToken());
     glyphStore->setURL(glyphURL);
 
-    for (const auto& source : style->sources) {
-        source->load(data.getAccessToken(), env, [this]() {
-            assert(Environment::currentlyOn(ThreadType::Map));
-            triggerUpdate();
-        });
-    }
+    resourceLoader = util::make_unique<ResourceLoader>();
+    resourceLoader->setAccessToken(data.getAccessToken());
+    resourceLoader->setObserver(this);
+    resourceLoader->setStyle(style.get());
 
     triggerUpdate(Update::Zoom);
 }
@@ -299,4 +299,9 @@ void MapContext::onLowMemory() {
     view.invalidate([this] { render(); });
 }
 
+void MapContext::onTileDataChanged() {
+    assert(Environment::currentlyOn(ThreadType::Map));
+    triggerUpdate();
+}
+
 }