The Vulkan back-end needs the layer count to correctly initialize the
whole range of the mip level. Otherwise we were only clearing a single
layer of the 2D array. This only popped up when the code started to be
lazier about robust resource init.

Bug: angleproject:4517
Change-Id: Id5007c877d413dc80d54d9800a5898e130593451
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2134412Reviewed-by: Cody Northrop <cnorthrop@google.com>