Consider adding flag for KHR_materials_common

[donmccurdy]

Similar to -k in COLLADA2GLTF, which an upcoming PR may rename to -m.

Some A-Frame users have been surprised when lighting/shadows do not work, and three.js has a bug (fixed but not yet released) with glTF custom shaders used in VREffect, so the common materials are preferable in several cases.

[lilleyse]

In a bit of a roundabout way this is supported in #49 with the --bypassPipeline flag. obj2gltf works with HKR_materials_common by default, but then sends it through gltf-pipeline to create techniques/shaders. This can be bypassed with that flag.

An explicit option for this seems like a good idea though.

[lilleyse]

A --kmc flag is added in #49 with a few caveats right now that would need fixing in gltf-pipeline:

gltf-pipeline uses the same kmc options for each material and doesn't recognize the transparent flag

[donmccurdy]

The new flag looks good, thanks!

[lilleyse]

Another comment from the code to add here and keep track of:

It's not completely clear whether transparent and doubleSided belong under values or KHR_materials_common
Put under both for now to handle both situations.
https://github.com/KhronosGroup/glTF/tree/master/extensions/Khronos/KHR_materials_common
KhronosGroup/glTF#632

[lilleyse]

Now in master with the --materialsCommon flag.