Added simplified off-line rendering API

Added calls a2_Render() and a2_RenderWave().

Added property A2_POFFLINEBUFFER, allowing applications to set
the default buffer size used for substates (when no driver is
specified), and the aforementioned calls.

Added three properties A2_PSILENCE*, allowing applications to
tweak the silence detection behavior (when 'length' == 0) of the
aforementioned calls and any future silence detection logic.

Added renderwave2.c test/example, using a2_RenderWave().

This completes the second stage of #8.