These decoders have been moved into their own subfolders under the
extras/decoders folder:

  extras/decoders/libvorbis
  extras/decoders/libopus

In addition to being relocated, they have also been split into separate
.c/h pairs. They now work like a more conventional library. The
implementation of these libraries have also been decoupled from the
miniaudio implementation which means they depend only on the header
section of miniaudio.h now.

With this change the custom_decoder and custom_decoder_engine examples
have been updated. To compile these you now need to link in the
miniaudio_libvorbis.c and miniaudio_libopus.c files via your build
tool. For your own code, you can still include the .c files directly
into your code if you want to compile as a single translation unit.

This commit fixes a bug relating to nodes with a scheduled start/stop
time. Whether or not the node is considered started or stopped is being
incorrectly reported by `ma_node_get_state_by_time_range()`.

Another issue is fixed in `ma_node_read_pcm_frames()`, which is related
to the fix above, where the frame count can underflow thereby resulting
in a crash.

Public issue #969

This removes the dependency on querying the required input frame count
from the resampler. This should in turn enable future work to support
custom resamplers.

Public issue #965

This is infrastructure work for supporting custom resamplers for the
pitching and Doppler effects for sounds.

Public issue #965

Public issue #965

This makes it less error prone when new channel positions are added to
the enum.

Public issue #714