[AD] al_is_addon_initialized

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

Should we have functions like that? I possibly said in the past that I
don't think we need them, but complaints on #allegro made me change my
mind. Basically there's situations where it would be somewhat awkward
having to keep track of the addons with global variables in user code
and so it gets hard to use the al_init_*_addon and al_shutdown_*_addon
functions. We already have al_is_system_installed and
al_is_audio_installed as well.

My naming suggestion:

al_is_acodec_addon_initialized
al_is_image_addon_initialized
al_is_font_addon_initialized
al_is_ttf_addon_initialized
al_is_primitives_addon_initialized

Also, maybe al_install_audio should be renamed to al_init_audio_addon
for consistency? (We can keep al_install_audio around as a #define like
we did for some other things in order to keep some 5.0 compatibility.)

-- 
Elias Pschernig <elias.pschernig@xxxxxxxxxx>