add export layers, export png, import brush padding

src/image.h

@@ -31,6 +31,10 @@ public:
         m_data.reset();
     }
     void flip();
+    // convert to grayscale and set opaque
+    void gayscale_alpha();
     void create() { m_data = std::make_unique<uint8_t[]>(size()); }
     Image resize(int w, int h) const;
+    Image resize_power2() const;
+    Image resize_squared() const;
 };