add first (_very rudimentary, but working) underflow prevention
add first (_very rudimentary, but working) underflow prevention

Required for supporting audio APIs that NEVER provide minimal frame size
requirements, f.e. the one selected on Windows 10.

currently only implemented for the libsoundio backend, and not thread
safe (for multiple backend instantiations, which is probably not an
important use case currently however);
The "correct" implementation would make this another "hidden data"
section, but mandatory (in contrast to the ones currently) even for
provided-buffer, backend-specific instantiation.
  (-> provide STRONGLY DISCOURAGED macro flag to disable)

Because the current implementation already _works, it should be merged
into master asap. Still TODO until then: Disable/ Warn about (or
implement) known failures, like multiple concurrent libsoundio backends.
rohlem/notelib,rohlem/notelib