Change all direct accesses to ALLEGRO_BITMAP::format to al_get_bitmap_format.

In many cases this isn't actually necessary (as we are guaranteed to be dealing
with parent bitmaps) but:

1. Not using al_get_bitmap_format invites human error in case that function is
   refactored
2. It is a premature optimization
Change all direct accesses to ALLEGRO_BITMAP::flags to al_get_bitmap_flags.

See previous commit for the rationalle.
Simplify al_convert_bitmap

In particular, the sub-bitmaps are now not touched, and just the parent is
converted.
Update the documentation for al_convert_bitmap
Rename ALLEGRO_BITMAP::flags, format to _flags and _format.

This highlights their private nature and will raise obvious errors in case some
place still accesses them directly. The leading underscore just seemed like a
good convention.
Give the D3D backbuffer bitmap a valid flag