Add an attachment helper to replace getRenderTarget.

We would like to stop exposing rx::RenderTarget to any GL-layer
code, like the Context, or Framebuffer. Then we can pull any
necessary information inside the Renderer back-ends.

BUG=angle:732

Change-Id: Ifa35218f097c528e616d732bda4bcd8e5f1c3843
Reviewed-on: https://chromium-review.googlesource.com/217029
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
diff --git a/src/libGLESv2/Framebuffer.h b/src/libGLESv2/Framebuffer.h
index 8d7678c..70998ff 100644
--- a/src/libGLESv2/Framebuffer.h
+++ b/src/libGLESv2/Framebuffer.h
@@ -113,4 +113,12 @@
 
 }
 
+namespace rx
+{
+class RenderTarget;
+
+// TODO: place this in FramebufferD3D.h
+RenderTarget *GetAttachmentRenderTarget(gl::FramebufferAttachment *attachment);
+}
+
 #endif   // LIBGLESV2_FRAMEBUFFER_H_