compute tip scale when loading from brush tips, remove tip_width which was replaced by tip_scale

src/brush.h

@@ -24,7 +24,6 @@ public:
 
     glm::vec4 m_tip_color = { 0, 0, 0, 1 };
     glm::vec2 m_tip_scale = { 1.f, 1.f };
-    float m_tip_width = 1.f;
     float m_tip_size = 50;
     float m_tip_spacing = .25;
     float m_tip_flow = 1.f;