TI-84+ LCD driver

Implement (emit) on TI-84+ (for now)'s LCD screen.
Load range: 555-560

The screen is 96x64 pixels. The 64 rows are addressed directly
with CMD_ROW but columns are addressed in chunks of 6 or 8 bits
(there are two modes).

In 6-bit mode, there are 16 visible columns. In 8-bit mode,
there are 12.

Note that "X-increment" and "Y-increment" work in the opposite
way than what most people expect. Y moves left and right, X
moves up and down.
                                                        (cont.)