output: add wlr_output_get_primary_formats
This allows compositors to get primary formats without manually calling wlr_output_impl.get_primary_formats.
For example, the Sway patch for linux-dmabuf feedback 1 needs this.
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This allows compositors to get primary formats without manually calling wlr_output_impl.get_primary_formats.
For example, the Sway patch for linux-dmabuf feedback 1 needs this.