Support VBLANK by floating bus

[bradgrantham]

Emulate the video scanout access pattern and floating bus so that the most frequently used programs trying to find VBLANK work.

Examples include: http://www.deater.net/weave/vmwprod/split_demo/ http://www.deater.net/weave/vmwprod/fancy_lores/

But these demos will not look correct without #27

[bradgrantham]

Note that Vince (deater.net/weave) uses green in GR (not HGR) to find VBL.

[bradgrantham]

Reference material:

• Understanding the Apple II
• Vince's split-screen demo source
• Bob Bishop's original Apple Split article from 1982
• Rainbow floating-bus example
• Vertical blank and mode change RetroChallenge entry