Return a special bitmap representing the back-buffer of the display.
Care should be taken when using the backbuffer bitmap (and its
sub-bitmaps) as the source bitmap (e.g as the bitmap argument to
Only untransformed operations are hardware accelerated.
This consists of al_draw_bitmap(3) and al_draw_bitmap_region(3) when the
current transformation is the identity.
If the tranformation is not the identity, or some other drawing
operation is used, the call will be routed through the memory bitmap
routines, which are slow.
If you need those operations to be accelerated, then first copy a region
of the backbuffer into a temporary bitmap (via the al_draw_bitmap(3) and
al_draw_bitmap_region(3)), and then use that temporary bitmap as the