[AD] Feature backports for 5.0.6

[Peter Wang <novalazy@xxxxxxxxxx>]

Hi,

The number of bug fixes to be backported to 5.0.6 is fairly low, so is a
good time to finally backport some small features.
Here are some proposals.  Let me know what you think and if you have
other suggestions.

Provisional yes
---------------

- al_load_ttf_font_stretch
- al_load_ttf_font_stretch_f

- al_draw_elliptical_arc
- al_draw_pieslice
- al_draw_filled_pieslice

- al_set_exe_name

- al_draw_tinted_scaled_rotated_bitmap_region
    silly name

- al_reset_clipping_rectangle
    trivial to work around

- al_register_assert_handler

- file slices
    ready?

- mixer gain

Dubious
-------

- ALLEGRO_SRC_COLOR, ALLEGRO_DST_COLOR,
  ALLEGRO_INVERSE_SRC_COLOR, ALLEGRO_INVERSE_DST_COLOR
    probably too much effort

- al_pause_event_queue, al_is_event_queue_paused
    not that useful; might delete it even

- al_add_win_window_callback, al_remove_win_window_callback
    not enough experience with it?