Fix depth clamping during 'very' early depth test.

Fix depth clamping during 'very' early depth test.

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/13088
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>