Oops, the previous commit swapped the main ega method with the main
vga planar method (for testing that was supposed to be local that the
former still works). The ega method works on vga but is about twice
as slow. The vga method doesn't work on ega.

Optimize the main vga planar method a little. For changing the
background color (which was otherwise optimized better than most
things), don't switch the write mode from 3 to 0 just to select
the pixel mask of 0xff obscurely by writing 0. Just write 0xff
directly.