Make al_destroy_bitmap REALLY maintain the current display when
untargeting the bitmap to be destroyed.

This was previously incorrect for memory bitmaps.
speed: Don't need a4_font.tga any longer.
speed: Draw circles with 0.5 offset from integer coordinates.

Also use thickness 1 instead of 0 for circle outline.
ex_audio_timer: Add new example.
pulseaudio: Minor cleanups.
pulseaudio: Clean up state transitions.
pulseaudio: Use smaller buffer size by default, and make it configurable.

The old default of 4096 was lags badly in ex_audio_timer.
I considered generalising it to an audio.buffer_size setting
but I'm not sure that it make sense.