save presets using serializer

src/brush.h

@@ -2,8 +2,9 @@
 #include "rtt.h"
 #include "shader.h"
 #include "texture.h"
+#include "serializer.h"
 
-class Brush
+class Brush : public Serializer::Type
 {
     std::shared_ptr<Image> m_tip_img;
     std::shared_ptr<Image> m_pattern_img;
@@ -101,6 +102,9 @@ public:
     bool preload();
     void unload();
     bool valid();
+
+    virtual bool read(BinaryStreamReader& r) override;
+    virtual void write(BinaryStreamWriter& w) const override;
 };
 
 struct StrokeSample