TI-84+ LCD driver

Implement (emit) on TI-84+ (for now)'s LCD screen. Lives at
B350.

Required config:

* LCD_MEM: 2b area where a that will point to an area allocated
  to LCD driver memory during LCD$ init.

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.

Z-Offset

This LCD has a "Z-Offset" parameter, allowing to offset rows on
the screen however we wish. This is handy because it allows us
to scroll more efficiently. Instead of having to copy the LCD
ram around at each linefeed (or instead of having to maintain
an in-memory buffer), we can use this feature.

The Z-Offset goes upwards, with wrapping. For example, if we
have an 8 pixels high line at row 0 and if our offset is 8,
that line will go up 8 pixels, wrapping itself to the bottom of
the screen.

The principle is this: The active line is always the bottom
one. Therefore, when active row is 0, Z is FNTH+1, when row is
1, Z is (FNTH+1)*2, When row is 8, Z is 0.

6/8 bit columns and smaller fonts

If your glyphs, including padding, are 6 or 8 pixels wide,
you're in luck because pushing them to the LCD can be done in a
very efficient manner.  Unfortunately, this makes the LCD
unsuitable for a Collapse OS shell: 6 pixels per glyph gives us
only 16 characters per line, which is hardly usable.

This is why we have this buffering system. How it works is that
we're always in 8-bit mode and we hold the whole area (8 pixels
wide by FNTH high) in memory. When we want to put a glyph to
screen, we first read the contents of that area, then add our
new glyph, offsetted and masked, to that buffer, then push the
buffer back to the LCD. If the glyph is split, move to the next
area and finish the job.

That being said, it's important to define clearly what CURX and
CURY variable mean. Those variable keep track of the current
position *in pixels*, in both axes.

Words descriptions

LCD_BUF: two pixel buffers that are 8 pixels wide (1b) by FNTH
pixels high. This is where we compose our resulting pixels
blocks when spitting a glyph.