ImageIndex: Consolidate layer/cube face.

In terms of the Texture or Image resource, a cube face
refers to a layer of a 2D texture. This layer has a special
meaning for cube textures, but it is represented as a layer
with a layer index. Cube array textures are no different,
they just use a different indexing scheme for the array
layers.

This also cleans up the ImageIndex helper to have a class
structure with private data, and cleans up a few cases to
use generic Make functions and iterators where they were
setting properties of the index directly.

This will make it easier to have ImageIndexes address
entire levels of a Cube map in the future, and makes the
layer count logic in Vulkan cleaner.

Bug: angleproject:2318
Change-Id: Iea9842e233f974a9896282ca224cb001f7882bd1
Reviewed-on: https://chromium-review.googlesource.com/987525
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Image.cpp b/src/libANGLE/Image.cpp
index e0e40b0..bfdb926 100644
--- a/src/libANGLE/Image.cpp
+++ b/src/libANGLE/Image.cpp
@@ -26,7 +26,7 @@
 {
     if (eglTarget == EGL_GL_RENDERBUFFER)
     {
-        return gl::ImageIndex::MakeInvalid();
+        return gl::ImageIndex();
     }
 
     gl::TextureTarget target = egl_gl::EGLImageTargetToTextureTarget(eglTarget);
@@ -40,7 +40,7 @@
     else
     {
         ASSERT(layer == 0);
-        return gl::ImageIndex::MakeGeneric(target, mip);
+        return gl::ImageIndex::MakeFromTarget(target, mip);
     }
 }
 }  // anonymous namespace