Backport Bug 1984016: Scale down dithering range to avoid GPU specific rounding difference

⬅️ Backport Patchset

Bookkeeping

Issue(s)

• https://bugzilla.mozilla.org/show_bug.cgi?id=1984016

Merge Request(s)

Target Channels

• Alpha
• Stable
• Legacy

Notes

Seems a trivial fix that could reduce differences between GPUs (add a constant multiplication to shaders).

The only reason I see not to backport is to ask upstream if they can do it for us instead.

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