Fix removing of the mouse image in vga planar mode with 8x8 fonts, and
reduce hard-coded assumptions on font sizes so that the cursor size
can be more independent of the font size. Moving the mouse in the
buggy mode left trails of garbage.

The mouse cursor currently has size 9x13 in all modes. This can occupy
2x3 character cells with 8x8 fonts, but the algorithm was hard-coded
for only 2x2 character cells. Rearrange to hard-code only a maximum
cursor size (now 10x16) and to not hard-code in the logic. The number
of cells needed is now over-estimated in some cases.

2x3 character cells should also be used in text mode with 8x8 fonts
(except with large pixels, the cursor size should be reduced), but
hard-coding for 2x2 in the implementation makes it not very easy to
expand, and it practice it shifts out bits to reduce to 2x2.

In graphics modes, expansion is easier and there is no shifting out
for 9x13 cursors (but 9 is a limit for hard-coding for 2 8-bit VGA
cells wide). A previous commit removed the same buggy hard-coding for
removal at a lower level in planar mode. Another previous commit fixed
the much larger code for lower-level removal in direct mode; this is
independent of the font size so worked for 8x8 fonts. Text mode always
depended on the higher-level removal here, and always worked since
everything was hard-coded consistently for 2x2 character cells.