[core] custom layer - add contextLost callback

- optional callback method that can be used to do cleanup when the context has been lost before re-initialisation

include/mbgl/style/layers/custom_layer.hpp

@@ -38,6 +38,14 @@ struct CustomLayerRenderParameters {
  */
 using CustomLayerRenderFunction = void (*)(void* context, const CustomLayerRenderParameters&);
 
+/**
+ * Called when the system has destroyed the underlying GL context. The
+ * `CustomLayerDeinitializeFunction` will not be called in this case, however
+ * `CustomLayerInitializeFunction` will be called instead to prepare for a new render.
+ *
+ */
+using CustomLayerContextLostFunction = void (*)(void* context);
+
 /**
  * Destroy any GL state needed by the custom layer, and deallocate context, if necessary. This
  * method is called once, from the main thread, at a point when the GL context is active.
@@ -51,8 +59,16 @@ public:
     CustomLayer(const std::string& id,
                 CustomLayerInitializeFunction,
                 CustomLayerRenderFunction,
+                CustomLayerContextLostFunction,
                 CustomLayerDeinitializeFunction,
                 void* context);
+
+    CustomLayer(const std::string& id,
+                CustomLayerInitializeFunction,
+                CustomLayerRenderFunction,
+                CustomLayerDeinitializeFunction,
+                void* context);
+
     ~CustomLayer() final;
 
     // Visibility

src/mbgl/renderer/layers/render_custom_layer.cpp

@@ -16,8 +16,12 @@ RenderCustomLayer::RenderCustomLayer(Immutable<style::CustomLayer::Impl> _impl)
 
 RenderCustomLayer::~RenderCustomLayer() {
     assert(BackendScope::exists());
-    if (initialized && !contextDestroyed && impl().deinitializeFn) {
-        impl().deinitializeFn(impl().context);
+    if (initialized) {
+        if (contextDestroyed && impl().contextLostFn ) {
+            impl().contextLostFn(impl().context);
+        } else if (!contextDestroyed && impl().deinitializeFn) {
+            impl().deinitializeFn(impl().context);
+        }
     }
 }
 

src/mbgl/style/layers/custom_layer.cpp

@@ -8,9 +8,18 @@ namespace style {
 CustomLayer::CustomLayer(const std::string& layerID,
                          CustomLayerInitializeFunction init,
                          CustomLayerRenderFunction render,
+                         CustomLayerContextLostFunction contextLost,
                          CustomLayerDeinitializeFunction deinit,
                          void* context)
-    : Layer(makeMutable<Impl>(layerID, init, render, deinit, context)) {
+        : Layer(makeMutable<Impl>(layerID, init, render, contextLost, deinit, context)) {
+}
+
+CustomLayer::CustomLayer(const std::string& layerID,
+                         CustomLayerInitializeFunction init,
+                         CustomLayerRenderFunction render,
+                         CustomLayerDeinitializeFunction deinit,
+                         void* context)
+    : Layer(makeMutable<Impl>(layerID, init, render, nullptr, deinit, context)) {
 }
 
 CustomLayer::~CustomLayer() = default;

src/mbgl/style/layers/custom_layer_impl.cpp

@@ -6,12 +6,14 @@ namespace style {
 CustomLayer::Impl::Impl(const std::string& id_,
                          CustomLayerInitializeFunction initializeFn_,
                          CustomLayerRenderFunction renderFn_,
+                         CustomLayerContextLostFunction contextLostFn_,
                          CustomLayerDeinitializeFunction deinitializeFn_,
                          void* context_)
     : Layer::Impl(LayerType::Custom, id_, std::string()) {
     initializeFn = initializeFn_;
     renderFn = renderFn_;
     deinitializeFn = deinitializeFn_;
+    contextLostFn = contextLostFn_;
     context = context_;
 }
 

src/mbgl/style/layers/custom_layer_impl.hpp

@@ -14,6 +14,7 @@ public:
     Impl(const std::string& id,
          CustomLayerInitializeFunction,
          CustomLayerRenderFunction,
+         CustomLayerContextLostFunction,
          CustomLayerDeinitializeFunction,
          void* context);
 
@@ -22,6 +23,7 @@ public:
 
     CustomLayerInitializeFunction initializeFn = nullptr;
     CustomLayerRenderFunction renderFn = nullptr;
+    CustomLayerContextLostFunction contextLostFn = nullptr;
     CustomLayerDeinitializeFunction deinitializeFn = nullptr;
     void* context = nullptr;
 };