vo_opengl: make :srgb decompand the BT.709 values correctly

This is the same issue as addressed by 257d9f1, except this time for
the :srgb option as well. (257d9f1 only addressed :icc-profile)

The conditions of the srgb_compand mix() call are also flipped to
prevent an off-by-one error.
vo_opengl: enable black point compensation in LittleCMS

This affects relative colorimetric mode only. In the future this might
be a user-exposed setting, but for now it'll just always be enabled when
using relative colorimetric mode.