vga: use latches in odd/even mode too

vga: use latches in odd/even mode too

Jazz Jackrabbit uses odd/even mode with 256-color graphics.  This is
probably so that it can do very fast blitting with a decent resolution
(two pixels, compared to four pixels for "regular" mode X).

Accesses still use all planes (reads go to the latches and the game uses
read mode 1 so that the CPU always gets 0xFF; writes use the plane mask
register because the game sets bit 2 of the sequencer's memory mode
register).  For this to work, QEMU needs to use the code for latched
memory accesses in odd/even mode.  The only difference between odd/even
mode and "regular" planar mode is how the plane is computed in read mode
0, and how the planes are masked if the aforementioned bit 2 is reset.

It is almost enough to fix the game.  You also need to honor byte/word
mode selection, which is done in the next patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>