Public issue https://github.com/mackron/miniaudio/issues/341

  * If starting the backend fails, don't attempt to enter the data loop
  * If stopping the backend fails, don't fire the stop callback.

Public issue https://github.com/mackron/miniaudio/issues/344

Public issue https://github.com/mackron/miniaudio/issues/341

This was happening because ma_log_level_to_string() was only being
included when MA_DEBUG_OUTPUT is enabled.

Fix `ma_log_postv` not returning anything under certain circumstances

The `__STDC_VERSION__ >= 199901L` and the `_MSC_VER >= 1200` code
had different styles for handling errors, so I've made them match.

MSVC 6 was printing a warning about this.

Public issue https://github.com/mackron/miniaudio/issues/340

Fix invalid vsnprintf buffer size

Was performing a `sizeof` on a pointer, which causes a warning to be
generated.

This fixes an issue where excess channels are not heard when the sound
source has more channels than the listener.

This commit also fixes a bug where panning is incorrectly getting
applied to non-directional channels, such as mono and LFE channels.

This is work towards using the heap for storing per-channel data so we
can get rid of upper channel count limit and remove MA_MAX_CHANNELS or
at the very least stop MA_MAX_CHANNELS from affecting overall memory
usage.