Implement Loopy's v, t, x, w logic; only background renders in 4 colors
Pipeline bg tile row data instead of just vram addresses, like on NES, and implement fine X scroll
Render background colors correctly

Compute high palette bits from attribute byte for each tile, then use with low palette bits to index into palette.
Clean/refactor for clarity
Include cycle 257 in HBlank/Idle phase which simplifies code and is more correct
Implement sprite rendering
Implement sprite vertical and horizontal flip, and fix overlapping sprite rendering
PPU fixes

* Hack fix $2002.7 (VBlank flag) polling never returning bit set before NMI executes. Fixes Bomberman and Burger Time.
* Respect render background and sprite flags ($2001.3 and $2001.4)
Merge branch 'correct-ppu-impl'