Handle 3D texture entire-mip initialization for FBO attachments 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/+/2174264 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>