This will make it easier to write YCbCr plane data as mipmap levels.

Also removes the unused fWidth, fHeight, and fDepth from sw::Mipmap,
and ensured the other members are aligned to their vector size.

Bug: b/132437008
Tests: dEQP-VK.*ycbcr*
Change-Id: Ib2b0cbebf9e4fd640fcc104281097ff3c4c73b5d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31616
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>