implement add and remove custom brushes

src/image.h

@@ -11,6 +11,7 @@ public:
     bool load_file(std::string filename);
     const uint8_t* data() const { return m_data.get(); }
     int size() const { return width * height * comp; }
+    bool save(const std::string& path);
     void create(int w, int h) 
     { 
         width = w;