vt1200.cpp declared PALETTE(config, m_palette).set_entries(256) with no init
delegate, so render_active_line()'s pens 0/1 rendered in whatever MAME's
default palette held -- reported as red on screen. Use
palette_device::MONOCHROME (emupal.cpp: palette_init_monochrome) with two
entries, so the colour is ours and deterministic.
Verified via a real MAME screenshot (screen:snapshot(), which renders through
the palette -- dump_frame()'s PPM is straight from bitmap pens and cannot show
colour): pal_mono_frame600.png is exactly black (0,0,0) + white (255,255,255).
palverify.lua takes the screenshot; the frame is the level-1 error screen.
