faster preview stages: cloning and interpolation

[hrj]

Two ideas to make previews faster:

• Copy only the decoded pixels into the preview image.
• Rearrangement and refactor of inner loops during interpolation

~Together, these give about 64% to 84% reduction in preview times.~ With these changes, the preview time is about 64% - 84% of earlier preview time.

For #333

[hrj]

Benchmark results using PolyFLIF (refer #333):

[saschanaz]

I think you meant (100-84)% to (100-64)% reduction, based on your last benchmark? Still impressive result :D

[hrj]

@SaschaNaz Yeah, my own wording confuses me a bit :smile: I meant to say, the time spent in preview is now 64% to 84% of the time spent previously.

[jonsneyers]

Very nice!