fix(linux/pipewire): try variable rate for non-KWin compositors - #5569

Description

Variable rate capture works better on mutter, but since GNOME 50
we encountered negotiation failures.

It seems that if we negotiate maxFramerate with a default of variable rate
but relaxed maximum value, the final negotiation phase actually lands on
variable rate (after being constrained to the host refresh rate); mutter
debug included:

[2026-08-29 00:38:43.671]: Info: [pipewire] Framerate (from compositor): 0/1
[2026-08-29 00:38:43.671]: Info: [pipewire] Framerate (from compositor, max): 60/1
SCREEN_CAST: Video format changed to 1920x1080 (framerate: 0/1 (max: 60/1))
SCREEN_CAST: Video format changed to 1920x1080 (framerate: 0/1 (max: 0/1))
[2026-08-29 00:38:43.672]: Info: [pipewire] Framerate (from compositor): 0/1 (variable rate capture)

Pacing does seem to be improved and Sunshine reports variable rate for the
final negotiation phase.

Upstream MR has been submitted, but doesn't seem strictly necessary:
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/5278

With upstream MR applied, output is more uniform:

[2026-08-29 00:35:42.248]: Info: [pipewire] Framerate (from compositor): 0/1 (variable rate capture)
[2026-08-29 00:35:42.248]: Info: [pipewire] Framerate (from compositor): 0/1 (variable rate capture)
[2026-08-29 00:35:42.248]: Info: [pipewire] Framerate (from compositor): 0/1 (variable rate capture)
[2026-08-29 00:35:42.248]: Info: [pipewire] Framerate (from compositor): 0/1 (variable rate capture)
SCREEN_CAST: Video format changed to 1920x1080 (framerate: 0/1 (max: 0/1))

Screenshot

Issues Fixed or Closed

Fixes #5000

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
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• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
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• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

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