Bug fix oiiotool --origin could crash with ImageCache-backed images.

In oiiotool's set_origin action, the simple resetting of the spec's x and y values is not safe if the IB is backed by ImageCache -- the mismatch of the image origin (which is related to the tile boundaries) between what the IC thinks and what the IB thinks leads to grief.  The fix is before resetting the image origin, cached images need to have a read forced to bring them into memory.  Images fully in memory already are fine to just diddle the origin value.

Fixes #791