When doing multisample rendering, we need to avoid conflicts between
render target writes to adjacent slices.

- Worst case is an unaligned 16-byte write hitting the last byte of the image, + 15 bytes beyond
- Ensure that slices are always 16-byte aligned

Test: dEQP-VK.renderpass.*
Bug: b/131719815
Change-Id: I5c3ee7c66f0f656fdc85d7388ca907e0173381e5
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/30608Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>