fix iOS deadlocks

WebAudio: add unlock audio context, fix default buffer size

  * ma_sound_set_start_time_in_milliseconds()
  * ma_sound_set_stop_time_in_milliseconds()
  * ma_sound_group_set_start_time_in_milliseconds()
  * ma_sound_group_set_stop_time_in_milliseconds()

  * ma_sound_set_fade_in_frames()       > ma_sound_set_fade_in_pcm_frames()
  * ma_sound_get_time_in_frames()       > ma_sound_get_time_in_pcm_frames()
  * ma_sound_set_start_time()           > ma_sound_set_start_time_in_pcm_frames()
  * ma_sound_set_stop_time()            > ma_sound_set_stop_time_in_pcm_frames()
  * ma_sound_group_set_fade_in_frames() > ma_sound_group_set_fade_in_pcm_frames()
  * ma_sound_group_get_time_in_frames() > ma_sound_group_get_time_in_pcm_frames()
  * ma_sound_group_set_start_time()     > ma_sound_group_set_start_time_in_pcm_frames()
  * ma_sound_group_set_stop_time()      > ma_sound_group_set_stop_time_in_pcm_frames()

This bug is due to the current time not being taken into account when
determining the node state.

Public issue https://github.com/mackron/miniaudio/issues/269