Prevent black screen on background resume in iOS MpvPlayer

[google-labs-jules]

Implemented proper lifecycle management for libmpv on iOS.
Modified enterBackground to set wid to 0.
Modified enterForeground to set wid back to the metalLayer.
Removed the workaround that disabled vid.

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PR created automatically by Jules for task 11242877970365080308 started by @Doezer

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[Doezer]

@jules what bug does it fix

[google-labs-jules]

@jules what bug does it fix

This change fixes an issue where the video output would remain black after returning to the app from the background. By properly detaching the window surface (wid=0) on background and re-attaching it on foreground, we ensure the player state is preserved and rendering resumes correctly, instead of just disabling the video track.