Flip in IOP flip and not upon loading

Notable changes in PR:
1. Upon loading image, it is not being rotated.
*This is needed for @hanatos dualiso branch - to keep black stripes in constant position*
2. Bitwise orientation is refactored into enum dt_image_orientation_t (MUCH easier to read)
3. IOP flip is enabled by default, and it does image rotation, that was previously done upon loading.