This makes the distinction between a gl::Extents (includes a depth
value for 2D array texture layer count) and a Vulkan extents (2D array
textures have a "1" for depth) clearer. Preparation refactor patch.

Bug: angleproject:3189
Change-Id: I9a13379c421e7f3c7856ac15b7a73013258ab9fe
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1709754Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>