This EGL extension will add support for creating EGLClientBuffer
backed by an Android window buffer (struct ANativeWindowBuffer)
which can be later used to create an EGLImage.

Bug: angleproject:5018
Tests: angle_end2end_tests --gtest_filter=ImageTest.SourceNativeClientBufferTarget*
Change-Id: If78ed7b80ad09629b8c5f5b5a0eb07a548e82e6e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2404320Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>