Prior to this change, RenderTargetVk had getImage() and
getImageForWrite().  This change introduces getImageForCopy() and
renames getImage() to getImageForRenderPass().

Currently, all three functions do the same thing.  However, with
upcoming changes the semantics will be different:

- getImageForRenderPass(): This is the image used as the non-resolve
  attachment.  When resolve attachments are introduced, there will be a
  corresponding getResolveImageForRenderPass().
- getImageForCopy(): When GL_EXT_multisampled_render_to_texture is
  implemented, this will return the image that owns the data.
- getImageForWrite(): Similar to getImageForCopy(), but should set
  mContentDefined.  This is currently missing, and is a bug that will be
  resolved in a follow up.

This split change gets the mechanical rename out of the way to make
future changes simpler.

Bug: angleproject:4836
Bug: angleproject:4859
Change-Id: I5f7657cc049c0e1772a7c510e74289e685ba93c3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2306516
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>