The 4 bytes starting from the 0x06 offset of a bmp file are marked as
'Reserved' but it doesn't guarantee that these bytes are zeroed, some
applications uses this area for its own purposes and the output file
still a valid bmp file.

The 'long' type can be 32 bits or 64 bits depending on what compiler
you are using (LLP64 vs LP64). If the long type is 64 bits the
biHeight variable will receive a very big number when the bmp image
height is negative, causing a std::bad_alloc exception when calling
the 'image.set_size' method.

why I didn't do this a long time ago.

hough_transform objects and made some argument names more consistent.

outputs.  Also cleaned up the code a bit.

rectangle.