OpenGL requires depth values to be clamped to the [0, 1] range. Due to
frustum clipping already limiting the range, this can only happen when
non-zero polygon offset parameters are active.

Also fix the 'minimum resolvable difference' for the 32-bit floating-
point internal depth format that we use.

Bug swiftshader:82

Change-Id: Ic9ebcac182a2bc81ab51d79cfe0bb451d340bd1e
Reviewed-on: https://swiftshader-review.googlesource.com/12108Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>