don't require the background layer to be named "background"

src/map/map.cpp

@@ -646,7 +646,7 @@ void Map::renderLayer(std::shared_ptr<StyleLayer> layer_desc, RenderPass pass) {
                 }
             }
         }
-    } else if (layer_desc->id == "background") {
+    } else if (layer_desc->type == StyleLayerType::Background) {
         // This layer defines the background color.
     } else {
         // This is a singular layer.

src/style/style.cpp

@@ -98,7 +98,7 @@ void Style::loadJSON(const uint8_t *const data) {
 const BackgroundProperties &Style::getBackgroundProperties() const {
     if (layers && layers->layers.size()) {
         const auto first = layers->layers.front();
-        if (first && first->id == "background") {
+        if (first && first->type == StyleLayerType::Background) {
             return first->getProperties<BackgroundProperties>();
         }
     }

src/style/style_layer.cpp

@@ -10,7 +10,7 @@ StyleLayer::StyleLayer(const std::string &id, std::map<ClassID, ClassProperties>
     : id(id), styles(std::move(styles)), rasterize(std::move(rasterize)) {}
 
 bool StyleLayer::isBackground() const {
-    return type == StyleLayerType::Background && id == "background";
+    return type == StyleLayerType::Background;
 }
 
 void StyleLayer::setClasses(const std::vector<std::string> &class_names, const timestamp now,