We perform depth testing before computing scanline invariants, as an
optimization. But it was not taking into account that the
fixed-function depth value may require clamping due to the depth bias.

Bug swiftshader:82

Change-Id: I6f8953f9d060c73b5fe209b8cbad70e7cb7588b4
Reviewed-on: https://swiftshader-review.googlesource.com/13088Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>