since they are expected to entirely fill it with data or silence.

For legacy apps that might expect the buffer to already have silence and thus
 may not fill the buffer in the callback, there's an environment variable to
 expose the old behaviour.

  Fixes Bugzilla #416.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402411