implement core egl image entry points Bug: angleproject:2467 Change-Id: Ica943a3972333e90a516aa6960f333cc9c378be2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1653714 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>