TRAC #11309

* Lift getRenderTarget() into base class Texture.
* Add pure virtual convertToRenderTarget() and getSurface().

Author:    Andrew Lewycky
Signed-off-by: Shannon Woods
Signed-off-by: Daniel Koch

git-svn-id: https://angleproject.googlecode.com/svn/trunk@54 736b8ea6-26fd-11df-bfd4-992fa37f6226