implement brush presets save and restore from file, fix stencil nullptr, limit preview stroke max size

src/brush.h

@@ -8,8 +8,10 @@ class Brush
 public:
     Brush() = default;
     Brush(const Brush& brush) = default;
-    int id = 0;
     std::string m_name;
+    std::string m_brush_path;
+    std::string m_brush_thumb_path;
+    std::string m_stencil_path;
     std::shared_ptr<Texture2D> m_tip_texture;
     std::shared_ptr<Texture2D> m_stencil_texture;
     //uint16_t m_tex_id = 0;
@@ -41,7 +43,7 @@ public:
     float m_jitter_sat = 0;
     float m_jitter_val = 0;
     int m_blend_mode = 0;
-    bool load_texture(const std::string& path);
+    bool load_texture(const std::string& path, const std::string& thumb);
     bool load_stencil(const std::string& path);
 };
 
@@ -94,6 +96,7 @@ public:
     float m_curve = 0;
     float m_dist = 0;
     float m_step = 0;
+    float m_max_size = FLT_MAX;
     Camera m_camera;
     std::shared_ptr<Brush> m_brush;
     cbuffer<float, 3> m_curve_angles;