More PPU changes.  I have an idea what's wrong.

Looking at .vnes.log, it would appear that the issue is within
the initial storing into name table memory.  I will try adding
frame count code to prevent rendering for the first X frames.
PPU now renders scanlines properly for nestest.nes

The issue has to do with not running the PPU for the first X
scanlines.  For now, the first three frames of a ROM don't render.