Various glTF fixes, mostly meshopt related

[kring]

EXT_meshopt_compression is fun because it is, as far as I'm aware, the only thing other than a regular bufferView that can reference a buffer. This wasn't being accounted for in a lot of the glTF manipulation code.

From the changelog:

• Added support for the following glTF extensions to Model::merge. Previously these extensions could end up broken after merging.
  • ...
  • EXT_meshopt_compression
• GltfUtilities::collapseToSingleBuffer now works correctly even if some of the buffers in the model have a uri property and the data at that URI has not yet been loaded. Such buffers are left unmodified.
• GltfUtilities::collapseToSingleBuffer now works correctly with bufferViews that have the EXT_meshopt_compression extension.
• GltfUtilities::compactBuffer now accounts for bufferViews with the EXT_meshopt_compression when determining unused buffer ranges.
• When GltfReader decodes buffers with data URLs, and the size of the data in the URL does not match the buffer's byteLength, the byteLength is now updated and a warning is raised. Previously, the mismatch was ignored and would cause problems later when trying to use these buffers.
• EXT_meshopt_compression and KHR_mesh_quantization are now removed from extensionsUsed and extensionsRequired after they are decoded by GltfReader.

Draft because I need to write tests for a lot of this, though I have empirical evidence in another project that these changes are necessary and that they are working well.

[kring]

I've addressed @lilleyse's comment and added tests, so this is now ready to review and merge.

[lilleyse]

The macOS runners are taking a while to launch. But everything else passed and the code looks good to me, so I'm going to merge.