GLTFExporter: Allow for 4-byte alignment requirement in processBufferView

[webprofusion-chrisc]

Along the same lines as #13220 this fixes validation for accessor offset by padding length to next 4-byte boundary.

[webprofusion-chrisc]

btw, this is in part to provide further support for facebook posts #13397

[mrdoob]

/ping @fernandojsg @takahirox

[fernandojsg]

We are computing the same value two times, on processBuffer https://github.com/mrdoob/three.js/blob/dev/examples/js/exporters/GLTFExporter.js#L276 and on processBufferView (as you proposed).
What about adding it as a parameter to processBuffer https://github.com/mrdoob/three.js/blob/dev/examples/js/exporters/GLTFExporter.js#L338 so we don't need to compute it again there?

[fernandojsg]

This is not needed after #12877
@mrdoob we could close it