Simplify 16-bit to 24-bit color mapping.
ili9340: refactor-ish changes to accepting data bytes.
ili9340: acceptColumnAddressByte
ili9340: acceptPageAddressByte
ili9340: emulate SC, EC, SP, EP registers correctly.

This reflects actual ILI9340 operation:

* CASET sets SC and EC registers (start col, end col).
* PASET sets SP and EP registers (start page, end page).
* RAMWR resets col = SC, page = SP.
* RAMWR enforces wrapping at EC and EP.

Go's SubImage, on the other hand, doesn't actually re-map coordinates as
the old code assumed, so it didn't work at all.