This feature is introduced by geometry shaders, where all the layers of
a texture can be attached to a framebuffer.  The geometry shader would
use gl_Layer to decide which layer the primitive should be rendered to.

Bug: angleproject:3571
Change-Id: Ib2ae8e227b226295f9e2f62f6b230839070bc95c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2582711
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>