CreateScreenshotUsingRenderTarget: Render to texture only when texture and camera are ready

[Popov72]

See https://forum.babylonjs.com/t/screenshot-with-post-process-not-includes-meshes/45168

Checking camera.isReady(true) make sure that post-processes are ready before going on.

[bjsplat]

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[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]

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