[Bug 1930982]

[Lsalzman]

Comment on attachment 9226932
Bug 1714511 - use vqmovun_s16 for packing pixels. r?sotaro

### Beta/Release Uplift Approval Request
* **User impact if declined**: Enabling Software-WebRender on ARM platforms can lead to visual artifacts. Should not impact x86 platforms or non-SW-WR configurations.
* **Is this code covered by automated tests?**: No
* **Has the fix been verified in Nightly?**: Yes
* **Needs manual test from QE?**: No
* **If yes, steps to reproduce**: 
* **List of other uplifts needed**: None
* **Risk to taking this patch**: Low
* **Why is the change risky/not risky? (and alternatives if risky)**: Software-WebRender on ARM is not deployed by default to release. Must be pref'd on. However, many users on tier-2 platforms like ARM/Linux are pref'ing the feature on, and it would be nice if we just supplied the fix to them so they don't need to manually apply it to their build.
* **String changes made/needed**:

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Title:
  Firefox 89: webrender breaks videos on Raspberry Pi

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