=== VLC (C9) overflow ===
If Number of Scan Lines (R9) is updated changed with a value less than the current VLC, then:
* on CRTCs 0/1/2, VLC overflows and will count up to its maximum value (31) before looping back
* on CRTCs 3/4, the current line is considered the last one of this CRTC character and VLC changes to 0 on the next line