Backport S/VGA timing fix from 86Box #13
Description
The off by one in PCem causes the refresh rate for S/VGA to be wrong (mainly relevant for DOS).
For example, standard DOS 70Hz (25.175MHz clock, 8 pixels per character, 95 Horizontal Total, 449 scanlines)
(25175000/((95+6)*8))/449 -> 69.3923791043Hz
(25175000/((95+5)*8))/449 -> 70.0863028953Hz