Vulkan: Rename 'extents' param to 'glExtents'. Merely a minor refactor to clean up the diff of a subsequent CL. In the follow up we will also distinguish the GL extents from the Vulkan extents/layerCount pair. Bug: angleproject:3189 Change-Id: Ibbb79bf2e6d3f798243b424bacbfb2e5538bfecf Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1709755 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>