Add extensions layered on top of GL_EXT_memory_object and GL_EXT_semaphore
to support GL & Vulkan interoperation on Fuchsia. They are analagous to
the file descriptor versions, except not quite as opaque.

This is a draft; the underlying vulkan extensions have not been submitted
to Khronos.

Bug: angleproject:3492
Change-Id: Ic4723ded9fff752c4186eaa6a7f07d9ac51a3364
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1642332
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>