This still uses the legacy approach with 6 pointers per mipmap level
for each cube face. Will be refactored to use a single pointer to access
faces as consecutive layers.

Bug: b/129523279
Tests: dEQP-VK.texture.filtering.cube.formats.r8g8b8a8_unorm.*
Change-Id: I1f59f121d2989e0ba79eb3441529b812a8d9111a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29969
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>