512 byte bootsector demo for the Apple II

I had more elaborate plans for this one, including having the duck 
appear randomly over the screen. Ran out of time and also out of bytes.

I also wanted a quacking noise, but couldn't figure out how to do that
in the bytes available.

This is an actual bootsector (well, Apple II has 256 byte disk sectors, 
but the disk boot ROM will let you specify more than one to load). Apple 
II loads the boot sector starting at $800, which is a problem for a 
Lo-res demo as that's also the location of lo-res PAGE2 graphics. So I 
had to waste a chunk of code to move the code out of the way before 
running it.

The spinning lo-res logo is actually using the hi-res shape table routines.
How is that possible?  It draws a small rotating copy of the logo to
the upper-left corner of the (not-displayed) hi-res screen. Then the code
copies this (while adding more colors) to the lo-res screen.  It's hard
to do this quickly, to save time and space it actually destroys the hi-res
image by shifting it down to zero while reading it out,
but this is actually good as then we don't have to clear the screen.
