Improving #MartyPC's video emulation with monitor emulation!
Instead of letting the CGA card's hsync and vsync outputs drive the display directly, we simulate a monitor that must lock on to these signals via vertical and horizontal PLLs.
This provides a semi-realistic 'bounce' when you change video-modes (needs a lil tweaking), but also means if you program an invalid video mode the screen will start to roll, like a real monitor.