Problem : in CoreAmstrad, output VGA resolution is already at top (1 pixel is 1 pixel), CRT 768x288 is rendered into a 800x600 VGA, so I can't add an filter (without destroying VGA signal). CoreAmstrad is not using a simple 384x288 scandoubler like other core in VGA.
Another sort of scanline does exist, one line out of two is black, or darker.
=== DONE : Monochrome option ===