Change parameters.currentLayer to index instead of layerIndex (#2956)

include/mbgl/gfx/drawable.hpp

@@ -151,9 +151,6 @@ public:
     /// Set sub-layer index
     virtual void setSubLayerIndex(int32_t value) { subLayerIndex = value; }
 
-    void setLayerIndex(int32_t value) { layerIndex = value; }
-    int32_t getLayerIndex() const { return layerIndex; }
-
     /// Depth writability for 2D drawables
     DepthMaskType getDepthType() const { return depthType; }
 
@@ -293,7 +290,6 @@ protected:
     DrawPriority drawPriority = 0;
     int32_t lineWidth = 1;
     int32_t subLayerIndex = 0;
-    int32_t layerIndex = 0;
     DepthMaskType depthType; // = DepthMaskType::ReadOnly;
     UniqueDrawableData drawableData{};
     gfx::VertexAttributeArrayPtr vertexAttributes;

include/mbgl/renderer/layer_group.hpp

@@ -169,12 +169,7 @@ public:
 
     void setStencilTiles(RenderTiles);
 
-    void updateLayerIndex(int32_t value) override {
-        layerIndex = value;
-        for (auto* drawable : sortedDrawables) {
-            drawable->setLayerIndex(value);
-        }
-    }
+    void updateLayerIndex(int32_t value) override { layerIndex = value; }
 
 protected:
     // When stencil clipping is enabled for the layer, this is the set
@@ -245,12 +240,7 @@ public:
 
     std::size_t clearDrawables() override;
 
-    void updateLayerIndex(int32_t value) override {
-        layerIndex = value;
-        for (auto& drawable : drawables) {
-            drawable->setLayerIndex(value);
-        }
-    }
+    void updateLayerIndex(int32_t value) override { layerIndex = value; }
 
 protected:
     using DrawableCollection = std::set<gfx::UniqueDrawable, gfx::DrawableLessByPriority>;

include/mbgl/renderer/render_target.hpp

@@ -62,6 +62,16 @@ public:
         }
     }
 
+    /// Execute the given function for each contained layer group in reversed order
+    template <typename Func /* void(LayerGroupBase&) */>
+    void visitLayerGroupsReversed(Func f) {
+        for (auto rit = layerGroupsByLayerIndex.rbegin(); rit != layerGroupsByLayerIndex.rend(); ++rit) {
+            if (rit->second) {
+                f(*rit->second);
+            }
+        }
+    }
+
     /// Upload the layer groups
     void upload(gfx::UploadPass& uploadPass);