Per the spec:

  If VkPipelineRasterizationStateCreateInfo::depthClampEnable is
  enabled, before the sample’s zf is compared to za, zf is clamped to
  [min(n,f),max(n,f)], where n and f are the minDepth and maxDepth depth
  range values of the viewport used by this fragment, respectively.

So when we read oDepth back from a shader that sets fragDepth, it should
only be clamped when depth clamping is enabled, and it should be clamped
to the viewport depth clamp values, not [0, 1].

Bug: b/184063472
Change-Id: I92e99b80f3929b8b8030f41f6a3afbfebda4e737
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/54408Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Commit-Queue: Sean Risser <srisser@google.com>
Tested-by: Sean Risser <srisser@google.com>