Two bugs are fixed in this change.

One is that framebuffer attachments to 3D textures should not attempt to
defer clears.  The clear staged for the 3D texture applies to all
slices, not just the slice the framebuffer is attached to (and that
would get cleared through deferred clears).

Secondly, when clearing an attachment to a 3D texture, the clear must be
applied immediately through a render pass loadOp to affect only the
slice that's attached.  This was already handled for layered
framebuffers where the clear was applied immediately if the 3D texture
render target had more layers than the framebuffer.  The condition for
this is generalized to check whether the 3D texture has more slices
(regardless of whether the render target is layered or not).

Test is based on one written by Austin Eng <enga@chromium.org>

Bug: angleproject:5967
Change-Id: I43cf5fc24673323eda8390021641e2238be6e375
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2923785Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>