Preview should not create unnecessary temp files

[davilimap]

When previewing transform, a temp file is always created for the preview. This should not happen if neither the source nor the transform file have not changed. The package should cache the latest temp file along with the timestamps of the original files. If the timestamps remain unchanged, just use the that temp file. If not, create another one.

[sayedihashimi]

I can't remember for sure, but I think XDT exposes enough APIs to enable the preview to happen in memory. So maybe it's possible to bypass the extra file all together.