Based on the design document, "The runtime will not clamp or validate
the input, but implementations may clamp to the range [0,1] if necessary.",
so we test for the EXT_depth_range_unrestricted extension, and only clamp if
it's not available (thus, necessary to do so).
NaNs are converted to zero as per "NaNs must be treated as 0, but the runtime
will convert NaNs to 0 on behalf of the implementation.", and a default bounds
are set to 0.0 and 1.0.
