* add a couple way of validating MaterialInstances BUGS=[333907416] * Update filament/include/filament/Engine.h Co-authored-by: Powei Feng <powei@google.com> * Update filament/include/filament/Engine.h Co-authored-by: Powei Feng <powei@google.com> --------- Co-authored-by: Powei Feng <powei@google.com>