text: Title: Bug in SURF sum_rect()
  Body: Shouldn't the lines 42 and 43 in file https://github.com/luispedro/mahotas/blob/master/mahotas/features/_surf.cpp be:
  
  ``` cpp
  y1 = std::min<int>(y1, integral.dim(0)-1);
  x1 = std::min<int>(x1, integral.dim(1)-1);
  ```
  
  ?
  