Inline version of set_color().
This is an inline version of set_color(), intended for use in the
vertical retrace simulator callback function (retrace_proc, which is now
If you really must use _set_color from retrace_proc, note that it should
only be used under DOS, in VGA mode 13h and mode-X. Some SVGA chipsets
aren't VGA compatible (set_color() and set_palette() will use VESA calls on
these cards, but _set_color() doesn't know about that).