util, modetest: add support for low-color frame buffer formats

Geert Uytterhoeven requested to merge geertu/drm:low-color into main

A long outstanding issue with the DRM subsystem has been the lack of support for low-color displays, as used typically on older desktop systems, and on small embedded displays.

This merge request adds support for color-indexed frame buffer formats with 2, 4, and 16 colors. It has been tested on ARAnyM using a work-in-progress Atari DRM driver.


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