Don't duplicate resources that are referenced multiple times

[lilleyse]

Fixes #416

Follow up of #474 which was closed. This has the same basic idea but extends it to work with separate resources.

[ViliusLuneckas]

Hi @lilleyse, any plans to merge it soon and release to npm registry?

[likangning93]

Thanks @lilleyse, just a couple small things:

[lilleyse]

@likangning93 all images, shaders, buffers now pass through writtenResourceMap, but yeah, ultimately only images and shaders will use it, not buffers.

This change handles all your comments and makes #483 (comment) OBE.

[likangning93]

Thanks @lilleyse!