Pixel: Increase frame rate

- Boost the SD card SPI rate to 16MHz.
- Stretch a frame until the previous one is displayed, or otherwise we
  get into a case where we keep changing the back-buffer without
  actually swapping.
Pixel: Only use the low-byte of the address port

This frees up the high byte of port D, which includes 3 of the grove
connectors (4, 5, 6) on the Pixel V2 board.
Pixel: Add missing pins owned by the RGB LED function

We previsouly forgot that the pins owned by the RGB LED function are
actually a super-set of the ones actually used, since the code does I/O
operations on "half-ports" as opposed to individual bits for performance
reasons. Now those pins are officially claimed.