Add a fast mode

[nathsou]

Add a fast mode for slower devices

Approaches:

PPU:

• Switch from pixel by pixel rendering to frame by frame rendering (TODO: handle mid-frame updates)
• Keep both rendered nametables in memory and only update a background tile when it changes
• Implement scrolling by sliding a window into the 2 frame buffers
• Cache rendered tiles

CPU:

• JIT compilation?