fix mono bitmaps

[stevesims]

create monochrome bitmaps using graphics foreground colour

also fixes logic for working out width in bytes of monochrome bitmaps. bitmaps are assumed to have a complete number of bytes per row, and start at the top-most bit. the width given is a pixel width, and does not have to be a multiple of 8. the final byte in a row will have pixels ignored if the width is not a multiple of 8.

@breakintoprogram - big apologies - I made this fix a month back, and it made it into the C8 VDP 2.1.0 release, but it completely passed me by that I had forgotten to raise a PR on Quark for it