GLTFLoader: Add renderer.gammaOutput=true to examples.

[donmccurdy]

Fixes #12554. See #11110.

[takahirox]

I don't perfectly understand gamma color topics yet, but renderer.gammaOutput should always be true when GLTFLoader is used? I'm wondering if we should write this note to document or somewhere.

[donmccurdy]

I'm still figuring this out too, but from #11110 it sounds like gammaOutput=true should be preferred most of the time, regardless of model formats, unless you are doing post-processing.

[donmccurdy]

Related: https://maddieman.wordpress.com/2009/06/23/gamma-correction-and-linear-colour-space-simplified/

[mrdoob]

Thanks!