There is no way to physically pick up the video blanking signal since it is
now buried inside the VGC chip. However, through software, there are two ways
to pick up this signal.
1. Arm the Vertical Interrupts as one would do with an Apple IIc.
2. Read the Vertical Blanking location at $C019 as one would do with
an Apple IIe.
Research the appropriate manual for how to do these operations. If a routine
is written to handle video timing for the Apple IIe by picking up video
blanking signals from an IO chip (UE5), the routine can be converted for
the IIGS environment with these two techniques.