These tests pass on Vulkan backend except on Nvidia cards.
Nivida vulkan driver does not clamp the depth fragment
in case of floating-point representation.

Bug: angleproject:3678
Test: dEQP-GLES3.functional.polygon_offset.float32_result_depth_clamp
      dEQP-GLES3.functional.polygon_offset.float32_factor_1_slope
Change-Id: I2be70f98aafac546af592aa55c5b56e836e53a46
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1824797Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>