Comparing two RGBA pixels with different RGB but Alpha=0 must be
true (because Alpha=0 means both color are totally transparent, RGB
values doesn't matter).
This problem was introduced in d732f5b05f
Now image_iterator.h and image_bits.h use forward declarations. And if
someone want to use the Image implementation, ImageBits, LockImageBits,
ImageIterators, or some function inside primitive_fast.h, they just have
to include image_impl.h file.