improving brush tips browser, average stroke direction

src/brush.h

@@ -58,6 +58,8 @@ struct StrokeSample
     bool valid() const
     {
         return !(
+            glm::isnan(angle) ||
+            glm::isinf(angle) ||
             glm::any(glm::isnan(col)) ||
             glm::any(glm::isnan(pos)) ||
             glm::any(glm::isnan(origin))
@@ -100,7 +102,7 @@ public:
     bool m_filter_points = true;
     Camera m_camera;
     std::shared_ptr<Brush> m_brush;
-    cbuffer<float, 3> m_curve_angles;
+    cbuffer<glm::vec2, 3> m_direction;
     cbuffer<float, 10> m_pressure_buff;
     cbuffer<glm::vec3, 3> m_hsv_jitter;
     StrokeSample m_prev_sample;