Add reading back of texture and saving result as DICOM
Add reading back of texture and saving result as DICOM

TODO:
* bug: not getting the correct attachment from readPixels
even though readBuffer is being called.
* zip file only downloads 3 dicom instead of 5
* when zipfile is too big saveAs causes browser crash (firefox)
* label result is saved as source SOP not SEG
* other issues with correctly annotating the dicom

pieper/dcmjs