Use ImageIndex in getRenderTarget.

Also change ImageIndex to allow invalid indexes for layer. In 3D
indexes, sometimes the layer parameter is invalid, indicating we
index an entire mip level instead of a layer of a mip level.

BUG=angle:741

Change-Id: I7b410f9aaf568f215bb93ae063934669a8fa09bd
Reviewed-on: https://chromium-review.googlesource.com/218312
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libGLESv2/FramebufferAttachment.cpp b/src/libGLESv2/FramebufferAttachment.cpp
index 7145e07..540ede1 100644
--- a/src/libGLESv2/FramebufferAttachment.cpp
+++ b/src/libGLESv2/FramebufferAttachment.cpp
@@ -140,6 +140,11 @@
     return mTexture.get();
 }
 
+const ImageIndex *TextureAttachment::getTextureImageIndex() const
+{
+    return &mIndex;
+}
+
 Renderbuffer *TextureAttachment::getRenderbuffer()
 {
     UNREACHABLE();
@@ -211,6 +216,12 @@
     return NULL;
 }
 
+const ImageIndex *RenderbufferAttachment::getTextureImageIndex() const
+{
+    UNREACHABLE();
+    return NULL;
+}
+
 Renderbuffer *RenderbufferAttachment::getRenderbuffer()
 {
     return mRenderbuffer.get();