Define all ImageSetEntry members out of line

This fixes compile errors in Windows DLL builds, where (due to
exceptions?) any of these members might need to call sk_sp's
destructor.

Change-Id: I82986afa4497216cda76b681782ccea3c530ba7a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207121
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>