Please add material extension for Bump Map

I know there is already a normal map. But let's face a true, a lot of 3D resources and 3D tools just like bump maps more.

glTF is not just for webGL, it is a perfect replacement of fbx for this PBR generation, for All 3D tools.

In that case, support bump map is a good choice.

For now, when exporting 3D model with Bump map from 3D tools, they won't convert bump map into normal map for users, they just put bump map into the Nomal map node.

And I believe they'll never do this converting thing for users

So, the best choice is supporting bump map as an extension of material.

Then all those 3D resources with bump maps can be converted into glTF nicely.

No files, tests, or entry points are named. Start by reviewing the existing normal-map material extension and glTF material-extension conventions, then define the bump-map extension's scope and representation. Done means the extension is specified clearly enough to support conversion of assets containing bump maps.

Written by the indexing model from the issue text.