disable brush start curve for other than iOS, frames capture for Windows, fix frame dt

engine/brush.cpp

@@ -195,8 +195,10 @@ void ui::Stroke::reset(bool clear_keypoints /*= false*/)
 }
 void ui::Stroke::add_point(glm::vec2 pos, float pressure)
 {
+#ifdef __IOS__
     m_curve = glm::min(m_curve + 0.1f, 1.f);
     pressure = pressure * glm::pow(m_curve, 2.f);
+#endif // __IOS__
     
     if (m_brush.m_tip_size_pressure)
         m_step = glm::max(m_brush.m_tip_spacing * m_brush.m_tip_size * pressure * 800.f, 1.f);