Stop using FramebufferAttachment::Target.

Target includes the binding (DEPTH/STENCIL/COLOR), which is not useful
for many operations. Simplify this to just passing the mip/layer.

This allows us to stop using this internal struct in other classes.

BUG=angleproject:1635

Change-Id: Ic5a11781bf45fe7835437fa1e363c190b876d453
Reviewed-on: https://chromium-review.googlesource.com/469152
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Renderbuffer.cpp b/src/libANGLE/Renderbuffer.cpp
index 7b30a46..68b011d 100644
--- a/src/libANGLE/Renderbuffer.cpp
+++ b/src/libANGLE/Renderbuffer.cpp
@@ -166,7 +166,7 @@
     return id();
 }
 
-Extents Renderbuffer::getAttachmentSize(const FramebufferAttachment::Target & /*target*/) const
+Extents Renderbuffer::getAttachmentSize(const gl::ImageIndex & /*imageIndex*/) const
 {
     return Extents(mWidth, mHeight, 1);
 }