Fix unused variable warning in get_base_iterator_buffer()

This fixes an unused variable warning which occurs in the
get_base_iterator_buffer() function when the base iterator
is not a buffer iterator and thus the iter argument is not
used.
Move adjacent_transform_iterator to detail namespace

This moves the adjacent_transform_iterator class to the
detail namespace.
Move binary_transform_iterator to detail namespace

This moves the binary_transform_iterator class to the
detail namespace.
Move pixel_input_iterator to detail namespace

This moves the pixel_input_iterator class to the detail
namespace.
Move swizzle_iterator to detail namespace

This moves the swizzle_iterator class to the detail
namespace.