Make underwater shadows work with refraction scale

Currently underwater shadows and refraction scale are mutually exclusive when set to anything not 1.0 as indicated by the warning here: https://gitlab.com/OpenMW/openmw/-/blob/master/docs/source/reference/modding/settings/water.rst#L98

Objective here is to fix the maths to allow for both underwater shadows and refraction scale. Then we could drop the check here: !340 (diffs)

Underwater Shadows was introduced with !340 (merged)