I'm pretty confused with how this all works. The VDP1 manual isn't terribly clear either.
At VDP1 initialization, go from 1-cycle mode to change-mode.
At the start of VBLANK-OUT (when the VBLANK-OUT IRQ is fired), set PTMR=0, VBE=0, and a FB swap will occur IF we set VBE=FCM=FCT=1
Poll until...