GLTFExporter: Some validation fixes

Docs: Improved TextureLoader page

Clarify variables in physics/terrain example to be more self documenting 

Fix logarithmicDepthBuffer bug

Video Pano Example: limit zooming to remove distortion

[DOCS] minor improvement of AnimationLoader doc

Examples: Limit zooming

Docs: Minor improvements

Docs: Update PDBLoader

Remove `flatShading` check from before r87

reference `FlatShading` directly; Use comparison result directly

Importing `FlatShading` from ./constants directly, instead of accessing it from `THREE`.

and `value === FlatShading` resolves to a boolean, so no need for a ternary operation