implement heightmap overlay for navigation

src/node_panel_grid.h

@@ -14,6 +14,23 @@
 #define NANORT_ENABLE_PARALLEL_BUILD
 #include "nanort.h"
 
+class NodeHeightmapOverlay : public NodeBorder
+{
+    NodeBorder* m_picker{ nullptr };
+    bool dragging = false;
+public:
+    glm::vec2 m_value{ 0.5f, 0.5f };
+    glm::vec2 m_old_value{ 0.f };
+    std::function<void(Node* target, glm::vec2 value)> on_value_changed;
+    virtual Node* clone_instantiate() const override;
+    virtual void clone_finalize(Node* dest) const override;
+    virtual void init() override;
+    void set_value(float x, float y);
+    virtual kEventResult handle_event(Event* e) override;
+    virtual void draw() override;
+    virtual void handle_resize(glm::vec2 old_size, glm::vec2 new_size) override;
+};
+
 class NodePanelGrid : public Node
 {
 public:
@@ -35,6 +52,7 @@ public:
     NodeButton* m_render;
     NodeButton* m_commit;
     HeightmapPlane m_hm_plane;
+    NodeHeightmapOverlay* m_hm_preview_nav;
     Sphere m_sphere;
     Image m_hm_image;
     Texture2D m_texture;