New graphics (glyphs) and small modification of do_PPU_updates.

Other minor updates: wait_vblank_setppumask. Relies on the appropriate command
being in PPU_buffer.
Mapper control, safer nmi routine

New routines:  mapper_write  and  swprgbank_save.  The first only does a  mapper
write (X  ->  $8000,  A  ->  $A000), and  the second uses  the first  and  write
which PRG banks are mapped. The NMI routine restores the PRG banks at the end of
execution. Also a note is added in Calling_conventions.
Began playing with sound.