Vulkan cubemaps use six consecutive layers for the faces, so we can
reuse the same addressing logic as for 2D array textures. Hence the
3D lookup vector becomes a 2D coordinate plus layer coordinate after
projection. The only difference is we don't have to clamp to the range
of layers.

This simplifies the sampled image descriptor since we only have to store
a single pointer per mipmap level. We also avoid the per-lane lookup
(gather) operation. YCbCr sampling was adjusted to not use the same
array of buffer pointers.

Also eliminate the unused lodOrBias parameter from computeLod*(). It's
added afterwards.

Bug: b/134164485
Bug: b/129523279
Change-Id: I5c349ff458aabb1d77e32104429b635d96237292
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31088Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>