vgabios: implement read char in graphics mode

vgabios: implement read char in graphics mode

GWBasic relies on this, so implement it to enable some serious retrocomputing.
There is no better way to do it than trying to match all characters one by one
against the current font.

This makes it possible to actually do something in SCREEN 1 and SCREEN 2
(without it, you can use graphics in the programs but not in direct mode).

I couldn't find documentation for what to return as the attribute, but
experimenting with DOSBox suggests 0 (and GWBasic accepts it).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>