Removed some extra overloads of TextureStorage::getRenderTarget and added a method to get the render target of a Texture3D layer.

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@2164 736b8ea6-26fd-11df-bfd4-992fa37f6226