implement stencil from file

src/brush.h

@@ -8,6 +8,7 @@ public:
     int id = 0;
     std::string m_name;
     uint16_t m_tex_id = 0;
+    uint16_t m_tex_stencil_id = const_hash("data/paper.jpg");
     glm::vec4 m_tip_color{1, 0, 0, 1};
     float m_tip_size = 0;
     float m_tip_spacing = 0;