Consolidate style::Layer properties API

The `style::Layer` class now exposes all the properties contained at `style::LayerImpl`.
This allowed to drop `style::Layer::accept()` method usage, avoid the repeated generated code
and thus save some binary size.

This patch is a part of the layers modularization effort.

src/mbgl/style/layers/raster_layer.cpp

@@ -42,40 +42,6 @@ std::unique_ptr<Layer> RasterLayer::cloneRef(const std::string& id_) const {
 void RasterLayer::Impl::stringifyLayout(rapidjson::Writer<rapidjson::StringBuffer>&) const {
 }
 
-// Source
-
-const std::string& RasterLayer::getSourceID() const {
-    return impl().source;
-}
-
-
-// Visibility
-
-void RasterLayer::setVisibility(VisibilityType value) {
-    if (value == getVisibility())
-        return;
-    auto impl_ = mutableImpl();
-    impl_->visibility = value;
-    baseImpl = std::move(impl_);
-    observer->onLayerChanged(*this);
-}
-
-// Zoom range
-
-void RasterLayer::setMinZoom(float minZoom) {
-    auto impl_ = mutableImpl();
-    impl_->minZoom = minZoom;
-    baseImpl = std::move(impl_);
-    observer->onLayerChanged(*this);
-}
-
-void RasterLayer::setMaxZoom(float maxZoom) {
-    auto impl_ = mutableImpl();
-    impl_->maxZoom = maxZoom;
-    baseImpl = std::move(impl_);
-    observer->onLayerChanged(*this);
-}
-
 // Layout properties
 
 
@@ -540,5 +506,9 @@ optional<Error> RasterLayer::setLayoutProperty(const std::string& name, const Co
     return Error { "layer doesn't support this property" };
 }
 
+Mutable<Layer::Impl> RasterLayer::mutableBaseImpl() const {
+    return staticMutableCast<Layer::Impl>(mutableImpl());
+}
+
 } // namespace style
 } // namespace mbgl