HOPKINS: Fix cursor tranparency

Before, cursor images were converted to 16 bpp, and anything that
was the same as the first color in PAL_PIXELS[] was made transparent.
Now, cursors images are drawn as 8 bpp with a cursor palette created
from PAL_PIXELS[]. This preserves all the black parts of the cursor
that weren't actually color index 0.

It would be nice if we only regenerated the cursor/palette when they
have actually changed, but that's for later.