Return a ref for FBO attachment image indexes.

This cleans up the syntax somewhat. Also place some more of the
helper methods in the header.

BUG=angleproject:963

Change-Id: I62419095a7b65486f54d74791027594e8e595e48
Reviewed-on: https://chromium-review.googlesource.com/263629
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/renderer/d3d/FramebufferD3D.cpp b/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
index 1a4734b..1d994ff 100644
--- a/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
+++ b/src/libANGLE/renderer/d3d/FramebufferD3D.cpp
@@ -380,9 +380,8 @@
         gl::Texture *texture = attachment->getTexture();
         ASSERT(texture);
         TextureD3D *textureD3D = GetImplAs<TextureD3D>(texture);
-        const gl::ImageIndex *index = attachment->getTextureImageIndex();
-        ASSERT(index);
-        return textureD3D->getRenderTarget(*index, outRT);
+        const gl::ImageIndex &index = attachment->getTextureImageIndex();
+        return textureD3D->getRenderTarget(index, outRT);
     }
     else if (attachment->type() == GL_RENDERBUFFER)
     {
@@ -425,9 +424,8 @@
         gl::Texture *texture = attachment->getTexture();
         ASSERT(texture);
         TextureD3D *textureD3D = GetImplAs<TextureD3D>(texture);
-        const gl::ImageIndex *index = attachment->getTextureImageIndex();
-        ASSERT(index);
-        return textureD3D->getRenderTargetSerial(*index);
+        const gl::ImageIndex &index = attachment->getTextureImageIndex();
+        return textureD3D->getRenderTargetSerial(index);
     }
     else if (attachment->type() == GL_RENDERBUFFER)
     {