videoTexture serialization

[RaananW]

this adds the needed functionality to serialize and parse a video texture.

src, isVideo and settings were missing. Until now VideoTexture was parsed as a regular source-less texture.

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
refs/pull/14393/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14393/merge/index.html

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https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14393/merge/index.html#WGZLGJ#4600

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