Backport fix 919ddb5 (Reflection point z plane)

[slizzered]

• The reflection point could be placed outside the gain medium, since the coordinates were always calculared
• Z-coordinate MUST be on one of the planes by definition, so we can set it there (when calculating it, there could be a VERY small difference to the real plane, but still a difference)
• since the file changed, simple cherry-picking of the commit didn't work
• full hash: 919ddb53beba92a6a3903a75e2ee13724ae4e16e

[erikzenker]

Good work, repetition tests can start :-)