opacity pressure and jitter, initial direction

src/canvas.h

@@ -131,7 +131,8 @@ class Canvas
     struct StrokeFrame
     {
         glm::vec4 col;
-        float pressure;
+        float flow;
+        float opacity;
         std::array<std::vector<vertex_t>, 6> shapes;
         glm::vec4 m_mixer_rect;
     };