code cleanup and improve brush direction

src/util.h

@@ -95,7 +95,7 @@ template<typename T, int N> struct cbuffer
         m_index = 0;
         m_count = 0;
     }
-    T& head()
+    const T& head() const
     {
         return m_index == 0 ? m_vec[m_count - 1] : m_vec[m_index - 1];
     }
@@ -118,6 +118,16 @@ template<typename T, int N> struct cbuffer
             tot += m_vec[i];
         return tot / (float)m_count;
     }
+    template<typename T2 = T> T2 average_threshold(T threshold) const
+    {
+        T2 tot{};
+        if (m_count == 0)
+            return tot;
+        int n = 0;
+        for (int i = 0; i < m_count; i++)
+            tot += glm::abs(m_vec[i] - head()) < threshold ? 0 : m_vec[i], n++;
+        return tot / (float)n;
+    }
 };
 
 template<typename T, int Max = 0>