Changes gl::RenderbufferStorage to rx::RenderbufferImpl

BUG=angle:782

Also eliminates Colorbuffer/DepthStencilBuffer and related friends
and adds the RenderbufferD3D implementation of RenderbufferImpl.

Change-Id: Ibe72c9589b3c238b952744e7cfd70d97ec898eb9
Reviewed-on: https://chromium-review.googlesource.com/222925
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>
diff --git a/src/libGLESv2/FramebufferAttachment.cpp b/src/libGLESv2/FramebufferAttachment.cpp
index 540ede1..894884a 100644
--- a/src/libGLESv2/FramebufferAttachment.cpp
+++ b/src/libGLESv2/FramebufferAttachment.cpp
@@ -187,7 +187,7 @@
 
 GLsizei RenderbufferAttachment::getSamples() const
 {
-    return mRenderbuffer->getStorage()->getSamples();
+    return mRenderbuffer->getSamples();
 }
 
 GLuint RenderbufferAttachment::id() const