focus: Save desired value, restore on mode change
This is intentionally not on top of https://source.puri.sm/Librem5/megapixels/-/merge_requests/23 because it is more general, and must work without it too.
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This is intentionally not on top of https://source.puri.sm/Librem5/megapixels/-/merge_requests/23 because it is more general, and must work without it too.