[AD] lines and rectangles now that we have the primitives addon

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

I'm wondering about the usefulness of al_draw_rectangle and
al_draw_line. They feel a bit out-of-place now with the primitives
addon. For one, the flags parameter seems useless - if the primitives
addon has no use for extra flags, the core functions certainly never
will. Also, as the primitives addon supports drawing to a memory buffer
and has no external dependencies, there's no reason we shouldn't use it
in our examples in place of the core functions. So my suggestion would
be:

Remove the current al_draw_line and al_draw_rectangle implementations,
and rename the corresponding _ex functions from the primitives addon.
Examples which draw lines and rectangles should then use the new
versions and depend on the primitives addon.

-- 
Elias Pschernig <elias@xxxxxxxxxx>