Fixed event leak when closing states.

a2_CloseAPI() was called before destroying the root voice along
with any remaining voices!
Fixed stupid pointer bugs in the "dummy" audio driver.
Doubled the "fbdelay" buffer size (TEMPORARY HACK)

Doubled the (still hardwired) size of the "fbdelay" buffers, just
so my songs so far will still play correctly at 96 kHz for now...
Static descriptors for built-in drivers, eliminating leaks

The descriptors for the built-in driver are now in a static
array, so there will be no memory leaks there no matter what!

Applications can still unregister some or all built-in drivers,
register user drivers etc; this is just a more robust
implementation in terms of avoiding irrelevant memory leak
warnings from Valgrind and similar.

Fixes #85.