util: add SMPTE pattern support for C2 format

util: add SMPTE pattern support for C2 format

Add support for drawing the SMPTE pattern in a buffer using the C2
indexed format.

As only four colors are available, resolution is halved, and the pattern
is drawn in a PenTile RG-GB matrix, using Floyd-Steinberg dithering.
The magnitude of the green subpixels is reduced, as there are twice as
many green subpixels as red or blue subpixels.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
---
Dithering example at https://drive.google.com/file/d/1g5O8XeacrjrC8rgaVENvR65YeI6QvmtO/view

v5:
  - Add Reviewed-by,

v4:
  - Replace FILL_COLOR() use by pentile_color_lut[],

v3:
  - Add Acked-by,

v2:
  - New.