Fix normals when mesh has both two-sided material and negative scale (#2375)
Fix normals when mesh has both two-sided material and negative scale (#2375)

* fix: flip back-facing normals for double-sided materials depending on their angle towards the eye

the original method of using gl_FrontFacing doesn't work for mirrored geometry

* refactor: rename normalSign to normalDirSign

Co-authored-by: Anders Gallon <e520a1931febe8eedfbfe4726916eebe7a6a775b@animech.com>
MicroWorldwide/PlayCanvas,playcanvas/engine,playcanvas/engine,MicroWorldwide/PlayCanvas