Trac #11396

Creates new depthstencils for new windows when multiple windows exist,
repurposes original depthstencil if old windows have all been destroyed.
Author: Shannon Woods
Signed-off-by: Nicolas Capens
Signed-off-by: Daniel Koch

git-svn-id: https://angleproject.googlecode.com/svn/trunk@28 736b8ea6-26fd-11df-bfd4-992fa37f6226