egl::Image destroys the source sibling before calling the backend destroy
function.  ImageVk needs the source image to do proper clean up.

BUG=angleproject:3722

Change-Id: I110d77a9c4151166624bb953c579102ac7dab6ee
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1709752Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>