Diffuse values about 1 ignored

Replaces https://github.com/KhronosGroup/glTF/issues/261

> looks like if a component is above 1.0 it ignores it. So in the example something that is bright white is actually drawn black because the diffuse component is never put into the gltf file. I would think the desired behavior would be to clamp negative numbers to 0.0 and numbers greater than 1 to 1.0.

Perhaps OBE with glTF 2.0 PBR?

Start by reproducing the linked diffuse-material example and trace how diffuse component values are exported to glTF. Check whether values above 1.0 are omitted and whether the proposed clamping behavior is still relevant for this converter or superseded by glTF 2.0 PBR. Done means the intended handling is confirmed and covered by a regression test.

Written by the indexing model from the issue text.