FramebufferAttachmentObject::initializeContents clears the entire mip
level for layered textures (not including cube map). This was done by
special casing 2D array and 2D multisample array textures but did not
work for 3D.

Generalize the logic to work for all layered texture types.

Handle clearing the entire 3D texture mip in Vulkan.

TEST=conformance2/misc/uninitialized-test-2.html

BUG=angleproject:4602

Change-Id: I35bb9fc3304f0553e8de68d205b0843845bf7549
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2174264Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>