New file audio.s, some code in it

It will contain the sound engine.
New OAM segment and OAM array var.

TODO: after just adding the "ready video" var, we could now write a tiny part
of PPU_updates().
do_PPU_updates implemented

Some graphics for testing are included.
What's in the TODO list is done!
Corrected latent bug in NMI

The bug didn't manifest itself yet, but as soon as some bankswitching would
be done in reset, it would crash hard.
New functions memset_vram and memcpy_vram