Made Image11::getStagingTexture private and removed the undefined Image11::updateStagingTexture method.

TRAC #13332

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

git-svn-id: https://angleproject.googlecode.com/svn/branches/dx11proto@1971 736b8ea6-26fd-11df-bfd4-992fa37f6226