refactor Brush to be used in shared_ptr

src/texture.h

@@ -9,6 +9,7 @@ class Texture2D
     GLint m_format = 0;
     GLint m_iformat = 0;
 public:
+    bool auto_destroy = false;
     bool has_mips = false;
     bool create(int width, int height, GLint internal_format = GL_RGBA8, GLint format = GL_RGBA, const uint8_t* data = nullptr);
     bool create(const Image& img);
@@ -22,6 +23,7 @@ public:
     bool ready() const { return m_tex != 0; }
     void create_mipmaps();
     glm::vec2 size() const;
+    ~Texture2D();
 };
 
 class Sampler