Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, assimp team".
Changed copyright end year to 2020 in every reference "Copyright (c) 2006-XXXX, ASSIMP Development Team".
Changed copyright end year to 2020 in LICENCE.rtf.
Changed copyright end year in CMakeFiles.txt files and any other places referencing Assimp with a copyright start and end year.
A problem that sneaked in from #1293 (f84851e893)
due to insufficient code review, later reported in #2059 but not fixed
properly. Having a white ambient practically means all other color information
is ignored and the model stays bright white no matter how you set up your
lighting, so putting it back to how it was before the commit above.
