[core] Introduce StyleLayer subclasses

src/mbgl/layer/background_layer.cpp

@@ -0,0 +1,12 @@
+#include <mbgl/layer/background_layer.hpp>
+
+namespace mbgl {
+
+RenderPass BackgroundLayer::applyStyleProperties(const float z, const TimePoint& now, const ZoomHistory& zoomHistory) {
+    applyTransitionedStyleProperty(PropertyKey::BackgroundOpacity, properties.opacity, z, now, zoomHistory);
+    applyTransitionedStyleProperty(PropertyKey::BackgroundColor, properties.color, z, now, zoomHistory);
+    applyStyleProperty(PropertyKey::BackgroundImage, properties.image, z, now, zoomHistory);
+    return properties.isVisible() ? RenderPass::Translucent : RenderPass::None;
+}
+
+}