Implemented offline rendering in scripts

Added 'wave' statement for rendering programs to waves at compile time.
The statement accepts a wave name followed by a body containing zero or
more attributes (wave flags, rendering parameters etc) and finally a
program call statement with optional arguments.

The 'length' argument to a2_Render() is now actually implemented...!

Added rendertest.a2s to test and demonstrate the above.

Added megastrings.a2s, demonstrating a "10k oscillator" strings sound
implemented through recursive offline rendering. (This needs some wave
features that aren't implemented yet, but it sort of works already.)

Updated some scripts that were using 'wave' as an argument name.

This completes stage three, and thus finally closes #8.