skip apple pencil first n samples to overcome the initial pressure spike

src/brush.h

@@ -96,6 +96,7 @@ public:
     cbuffer<glm::vec3, 3> m_hsv_jitter;
     StrokeSample m_prev_sample;
     std::vector<Keypoint> m_keypoints;
+    std::vector<std::pair<glm::vec2, float>> m_hold_points;
     std::vector<StrokeSample> m_samples;
     int m_last_kp;
     std::minstd_rand prng;