Image11 now stores an ID3D11Resource for the staging texture and creates 3D staging textures if depth is greater than one.

TRAC #22705

Signed-off-by: Jamie Madill
Signed-off-by: Shannon Woods
Author: Geoff Lang

git-svn-id: https://angleproject.googlecode.com/svn/branches/es3proto@2162 736b8ea6-26fd-11df-bfd4-992fa37f6226