tests/color-icc-output: add ICC VCGT tests

There are some ICC profiles that contain something named VCGT tag. These
are usually power curves (y = x ^ exp) that were loaded in the video
card when the ICC profile was created. So the compositor should mimic
that in order to use the profile.

Weston already has support for that, but our ICC profile tests were
missing this case. This adds such tests.

For testing purposes, we have added tests with different exponents per
color channel.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>