adding color picking on iOS

engine/canvas.h

@@ -65,6 +65,8 @@ public:
     RTT m_tmp[6];
     Texture2D m_tex[6];
     Texture2D m_tex2[6];
+    bool m_pick_ready[6];
+    std::unique_ptr<glm::u8vec4[]> m_pick_data[6];
     static glm::vec3 m_plane_origin[6];
     static glm::vec3 m_plane_normal[6];
     static glm::vec3 m_plane_tangent[6];
@@ -104,6 +106,9 @@ public:
     void stroke_cancel();
     void stroke_commit();
     void clear(const glm::vec4& color = { 1, 1, 1, 0 });
+    void pick_start();
+    void pick_update(int plane);
+    glm::vec4 pick_get(glm::vec2 canvas_loc);
     void snapshot_save(std::string data_path);
     void snapshot_restore();
     void snap_history(const std::vector<int>& planes);