Fix geometry.mergeMesh & merge check bug

Fix setViewport & setScissor pixelRatio bug

Update MeshStandardMaterial.html

Update Textures.html

Disambiguation re: https://github.com/mrdoob/three.js/issues/11367#issuecomment-306245201
Clearly describe the expected format of each type of cubemap image, and the resulting render.  The Cube*Mapping settings were detailed explicitly enough, but users coming from other contexts may know the other two by different names.  E.g. in the popular Unity and Unreal game engines, "spherical" reflection and HDRI maps implicitly mean lat-long.

Regarding https://github.com/mrdoob/three.js/issues/11452#issuecomment-306533171
Clarify requirements on the environment map for advanced use cases.

ColladaLoader2: Improve processing of visual scene

[glTF] Update all examples to glTF2.0

Update Useful-links.html

Removed an outdated link.

allow index attributes to be dropped using index.onUpload() mechanism.

Fixed document typo

SkeletonHelper: Reverted call removal. .boundingSphere gets computed before onBeforeRender() gets called.