Allow 30-bit (10, 10, 10) framebuffers

This really just allows 30 (in addition to 16 and 24) to be specified as
depth in an attempt to check whether hardware automatically uses a deep
color mode.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Fill README with contents

Provide some detailed build instructions and a short overview of how to
run test cases.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>