Consider a "global" decode matrix for WEB3D_quantized_attributes?

Just wanted to throw this out there, from the perspective of using glTF for CAD & BIM (feel free to close if this is just a sleep-deprivation brainfart ;).

The [WEB3D_quantized_attributes](https://github.com/KhronosGroup/glTF/tree/master/extensions/Vendor/WEB3D_quantized_attributes) extension specifies that each accessor be accompanied by its own decode matrix, which would mean that each separate geometry is encoded separately within its own Local-space extents, which is great for precision.

However, this also means that each geometry is potentially quantized/decoded with a (albeit slightly) different level of precision, which could be a problem for applications like CAD or BIM viewers that need rectilinear elements to be in perfect alignment with each other.

Perhaps there is a way to quantify all geometry uniformly, using the World-space extents of the entire model, where a global decode matrix would be supplied for the whole model. Not sure what the loss of precision would be, but then everything would line up perfectly once uniformly decoded.

Start with the WEB3D_quantized_attributes extension linked in the issue and review how each accessor's decode matrix is specified. Compare that approach with the proposed model-wide matrix for CAD and BIM alignment; done would require a decided specification change, but the issue contains no named files or tests.

Written by the indexing model from the issue text.