Fix clamping depth output to [0, 1] range.

Fix clamping depth output to [0, 1] range.

OpenGL ES 3.0 specifies that "For fixed-point depth buffers, the final
fragment depth written by a fragment shader is first clamped to [0, 1]
and then converted to fixed-point as if it were a window z value (see
section 2.13.1). For floating-point depth buffers, conversion is not
performed but clamping is."

Change-Id: Ic7f22f7e47106aaa86c4916fbbbe23ad9c52e186
Reviewed-on: https://swiftshader-review.googlesource.com/15628
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>