Fix oiiotool --autotrim to use all subimages

[lgritz]

Previously, --autotrim when applied to a multi-subimage file would trim
all subimages to the nonzero region of just the first subimage.

This fixes, now examining all subimages and trimming them all to the
union of their nonzero regions (as --trim always did).

Technically, a multi-part OpenEXR can trim each subimage ("part")
separately, each having a different data window. But in practice, many
reading applications (including RV and Nuke!) seem to have trouble
reading files like that. So we go with the union approch.