Use whole preview image when decoding QR

When scanning Mailbox QR, the whole preview is visible on screen, so we
should use it. We choose to never crop the preview, because it was
originally a speed optimization which no longer is needed.
4 jobs for !1600 with 2269-use-full-camera-preview-when-scanning-qr-codes in 9 minutes and 44 seconds (queued for 35 seconds)
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