Fix Marble Madness

[quephird]

This PR finally addresses a bug in disaffecting the display of the text boxes at the beginning of each level in Marble Madness; this is what it looked like previously:

After ruling out a ton of other possibilities, namely the implementations behind caching and rendering of tile data by the PPU, it turned out that it was a fairly trivial mapping issue. In the case of the Marble Madness game, it uses the singleScreen1 mirroring strategy and previously we were incorrectly computing actual nametable indices for it.

The fix was to update the mapping, this time representing it as a two-dimensional array instead of a switch/case statement. Also the mapping has been moved into the Mirroring enum, and an instance method added there to return the correct nametable index for a given mirroring strategy and inbound nametable index. And now everything renders perfectly:

Other fixes/updates included in this PR:

• Subtle bug fix in the reading PPU memory in the range 0x3F00-0x3FFF; we hadn't been properly setting the buffer value previously.
• We also changed the internal API a little bit so that readByteInternal() no longer returns a tuple.
• The PPU dump utility also had a few bugs that resulted in the output being incorrect.