According to the Vulkan spec:
"If pResolveAttachments is not NULL, each of its elements
 corresponds to a color attachment (the element in
 pColorAttachments at the same index), and a multisample
 resolve operation is defined for each attachment. At the
 end of each subpass, multisample resolve operations read
 the subpass’s color attachments, and resolve the samples
 for each pixel to the same pixel location in the
 corresponding resolve attachments, unless the resolve
 attachment index is VK_ATTACHMENT_UNUSED."

Note: This cl adds support for multisampling, but
      requires syncing before performing the resolve
      operation. The intent is for the next cl to move
      the resolve to Renderer::finishRendering(), in
      order to avoid having to sync when it's not
      necessary.

Bug b/119620965

Change-Id: Id4fae41347e354b822d089fb5b6d4e36592c146b
Tests: dEQP-VK.pipeline.multisample.raster_samples.*
Tests: dEQP-VK.pipeline.multisample.raster_samples_consistency.*
Tests: dEQP-VK.pipeline.multisample.sample_mask.*
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27650Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>