For xcf files, compose layers before scaling

Otherwise if some layers are a different size than the overall
image, the output will be distorted as all the layers will be
resized.

An example of such a file is the 2007 version of
[[File:Blason_Saint-Malo.xcf]] (Need a version of image magick
newer than production to see the issue).

Bug: 35622
Change-Id: Ibbe8645afc151b0e9b658f762497071d67be3836
Merge "For xcf files, compose layers before scaling"