This patch implements Mirror influence for environment textures. Approach matches the one from BI.
{F281871}
See the video https://youtu.be/BskgCv6dcIE
Example: {F281876}
Alexander (Blend4Web Team)
Reviewers: campbellbarton, merwin, brecht
Reviewed By: brecht
Subscribers: TwisterGE, blueprintrandom, youle, Evgeny_Rodygin, AlexKowel, yurikovelenov
Differential Revision: https://developer.blender.org/D1786