Prefer direct buffers when creating ByteBuffer instances

[javagl]

Brought up in #60 :

There are some places where a ByteBuffer is created that is not a direct ByteBuffer. In order to improve interoperability with native libraries that require direct buffers, buffers created by JglTF should preferably be direct buffers.

This will require some potentially breaking changes, for example, related to the IO#read methods. But it might be possible to keep these functions, mark them as deprecated, and remove them soon, because clients should usually not call the low-level IO methods anyhow.