[Cin] fileexr/fileppm direct copy support

[Andrew Randrianasulu]

So, I was working on restoring direct copy support for cinelerra-cv and
realized I can xcheck Cin-GG too.

I think attached two patches make it possible to skip de/encoding for
unchaged portions of timeline when working with image lists of exr and ppm
types (in addition to png/tiff/tga/jpeg support already there)

Please test?
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