Req: Embedded raw preview *if* available

[Marsu42]

As a lot of Adobe users, I have converted my raw files to .dng - and this format allows you to chose if you want to embed a jpeg preview or not.

Current behavior: if you choose "Camera RAW, use: embedded preview" dng files w/o preview display an unusable 256x171 thumbnail. On the other hand, if you choose "Camera RAW, use: half/full size" the embedded preview is ignored even if it exists at the cost of a lot of rendering time.

Requested behavior: Add a checkbox "prefer embedded preview if exist" next to the current raw option so the quickest full res rendering is done automatically.

Thanks for considering!