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

src/brush.cpp

@@ -339,6 +339,8 @@ bool Brush::load_tip(const std::string& path, const std::string& thumb)
     m_tip_texture->auto_destroy = true;
     m_brush_path = path;
     m_brush_thumb_path = thumb;
+    auto sz = m_tip_texture->size();
+    m_tip_scale = sz.y > sz.x ? glm::vec2(sz.x / sz.y, 1.f) : glm::vec2(1.f, sz.y / sz.x);
     return true;
 }