Filament 1.12.8 + Documentation and glTF Sample in Kotlin + Fixes Augmented Face + Optimizations

• Filament 1.12.0 -> 1.12.8
• Documentation and samples
  • Home samples moved to Kotlin
  • glTF Sample moved to kotlin (gtf-java for old java version)
  • Add "Remove an AnchorNode from the Scene"
  • Website visual changes
• Fixes:
  • Disable Light estimation when the front face camera is used because ARCore estimation only works with the back camera
• Performance
  • AugmentedFaces optimisation and generalization
  • Plain videos can now be shown again without glitches
  • Updated the Depth Material to work again with the latest Filament Version
  • Made small improvements for setting texture name in ArSceneView
• Cleanup
  • Use the current branch for all samples if build in debug mode
  • Cleanup build.gradle files