The new code is based on timestamps, rather than delays. This eliminates the
need to update every timer in timer_process(), which cost a non-trivial amount
of CPU time on low-end machines.
This involved changes to every user of the timer code, so this change almost
certainly introduces bugs.
* Enables [Fn]+key combinations (using right-Ctrl or right-Alt as [Fn]). In particular:
* Implements switching the built-in display adapter between the 'internal' plasma display ([Fn]+[Home]) and an 'external' CGA monitor ([Fn]+[End])
* Allows the video font to be toggled between thin and thick ([Fn]+[Right arrow]).
* Implements the four hardware fonts of the original, selectable in BIOS setup.
* Implements CRTC register 12h (attribute mapping and display type)
Patch from John Elliott
