Make DirectShow file open async and cancelable

[tufeixp]

For those slow streaming videos(and pictures), the opening process will freeze the Unity main thread and user can do nothing about it! It's a terrible user experience!

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale[bot]]

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