remove all async_start/end calls

src/node_stroke_preview.cpp

@@ -264,6 +264,7 @@ void NodeStrokePreview::draw_stroke_immediate()
     glm::mat4 ortho_proj = glm::ortho<float>(0, size.x, 0, size.y, -1, 1);
     glViewport(0, 0, m_rtt.getWidth(), m_rtt.getHeight());
     m_rtt.bindFramebuffer();
+    m_rtt.clear();
     m_sampler_mipmap.bind(0);
     m_sampler_linear.bind(1);
     m_sampler_linear_repeat.bind(2);