video-ram address and no double-buffering

- moved video-ram to $6000, so it doesn't interfere with C64 ROMs
- removed double-buffering code, not needed for this demo
removed all processor port RAM/ROM switches, no longer needed.
added some raster IRQ effect
opened up VIC-II upper and lower border

no visible effect, but needed to display sprites over the top and bottom bars
replaced the bottom bar by a little scrolling using 8 sprites
added flashing effect for the scroller
set stretch-x for scroller (doubled width)
scroller background

- X-stretching made the moving sprites pop off the screen before they're
  completely hidden by VIC border -> reverted
- placed the scroller on a bright-blue bar for nicer optics
interrupt-driven song playback

- moved sound playing routine in interrupt for exact timing
- used raster line interrupt for this to avoid interference with other raster
  effects