samples: video: capture: Enhance fixture test with colorbar checking

Currently, the camera fixture test can only tell whether data are dumped
onto memory. This is to check further if the image content is correct.

The algorithm is rather simple: compare the average color of each bar
of a generated pattern with a predefined target in CIELAB76 color
space (which is nearly perceptually uniform).

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>