Screenshot tools: Fallback on regular canvas if offscreen canvas not supported

[Popov72]

See https://forum.babylonjs.com/t/safari-create-screenshot-using-render-target-broken/42512/8

Please wait for comments from the author of the thread to make sure the fix works before merging it! Can be merged!

[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]

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/14059/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14059/merge/index.html

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