upper two bits in 16-bit character mode indicate width of card.
implement $FFFF as early end of row marker in 16-bit text mode.
change raster buffer to be 18 bits wide ready for hosting alpha
channel data for anti-aliased text.
fix some field width bugs for raster buffer.
implement alpha values for pixels, including using blink attribute
in 16-bit text mode to signal alpha mode in full-colour text mode.
implement alpha blender reference implementation from Xilinx.
begin integrating alpha blender.