limit camera zoom

src/canvas.h

@@ -138,7 +138,9 @@ public:
     Sampler m_sampler_mix;
     glm::mat4 m_cam_rot = glm::mat4(1);
     glm::vec3 m_cam_pos{ 0 };
-    float m_cam_fov = 85;
+    float m_cam_fov = 85.f;
+    const float m_cam_fov_min = 5.f;
+    const float m_cam_fov_max = 150.f;
     glm::vec2 m_cur_pos;
 
     std::shared_ptr<Brush> m_current_brush;