Content-defined tracking was done in render targets prior to this
change.  This had multiple drawbacks:

- When a framebuffer attachment is changed (including the first time
  it's set), it's unknown whether the contents of the attachment is
  defined.
- Invalidate takes effect at the end of render pass, at which point the
  render target objects may be gone.  Attachment ImageHelpers are
  however correctly tracked.

This change moves content-defined tracking to the ImageHelper itself,
and tracks it per subresource.  ImageHelper::onWrite() now receives the
subresource that is being written, and marks it as having defined
content.

A future optimization can make use of this change to
ImageHelper::onWrite to track "dirty" subresources.  This can lead to
the removal of unnecessary barriers when same-kind writes are done on
different subresources of the image.  See http://anglebug.com/3347#c15

Bug: b/167275320
Bug: angleproject:4836
Change-Id: Iabd1dace4eae9eb379453a9eb7ec6eafc9db1aef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2462036
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>