Some SampledImageDescriptor members are assumed to be 16-byte aligned,
such as the sw::Mipmap::pitchP member accessed by sampleFloat2D(). To
guarantee this while descriptor sets can contain multiple types of
descriptor, all descriptors are made a multiple of 16 bytes in size.

Also, each descriptor set starts with a pointer to its layout, followed
by the actual descriptor content. To ensure the latter is 16-byte
aligned, the layout pointer was placed in a header structure which
itself is 16-byte aligned and thus includes the necessary padding.

The placement of the descriptor set itself is guaranteed to be 16-byte
aligned by vk::REQUIRED_MEMORY_ALIGNMENT = 16 being used for descriptor
pool allocation.

Bug: b/129523279
Bug: b/123244275
Change-Id: I72fb92d9ff5a8d100d05f5b9e38170ade557d434
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29970
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>