Compute SVD only once for each uploaded image. This makes the app faster.

I created an 'global' object `lst` for SVD result storage.

 And I splitted `writeSVD` into two isolate functions: `runSVD` and `doRecovery`. I call `runSVD` when rendering the original image, and `doRecovery` when rendering the svd image. 

There's a significant difference of performance on my computer, comparing to the original version.