[AD] al_update_display_region

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

I wonder if it would make more sense to have it fall back to
al_flip_display in case that's all it can do. The way it works right
now, if you use this function, you basically always would have to use it
like this:

if (!al_update_display_region(...))
    al_flip_display();

Which is a bit bad design. With the change you would instead simply do,
for the same effect:

al_update_display_region(...);

For the scenario where you actually have to know whether the two
functions do something different (I couldn't think of a reason), we
could provide a display flag returned by al_get_display_flags() which
indicates whether the current display does support it.

-- 
Elias Pschernig <elias@xxxxxxxxxx>