10X speedup (#2779)

Co-authored-by: pf <pf@me>

dlib/sliding_buffer/circular_buffer.h

@@ -49,7 +49,10 @@ namespace dlib
                 << "\n\t i:      " << i 
                 << "\n\t size(): " << size()
             );
-            return data[(i+offset)%data.size()]; 
+            auto index = i + offset;
+            if (index >= data.size())
+                index -= data.size();
+            return data[index]; 
         }
 
         const T& operator[] ( unsigned long i) const 
@@ -61,7 +64,10 @@ namespace dlib
                 << "\n\t i:      " << i 
                 << "\n\t size(): " << size()
             );
-            return data[(i+offset)%data.size()]; 
+            auto index = i + offset;
+            if (index >= data.size())
+                index -= data.size();
+            return data[index];
         }
 
         void resize(unsigned long size)