Sample Shading and FragCoord interpolation not fully specified for Z and W.

With regard to FragCoord and sample shading, the standard says this:

> When Sample Shading is enabled, the x and y components of FragCoord reflect the location of one of the samples corresponding to the shader invocation.
>
> Otherwise, the x and y components of FragCoord reflect the location of the center of the fragment.
>
> The z component of FragCoord is the interpolated depth value of the primitive.
>
> The w component is the interpolated 1/w​.

This makes it clear that the XY components represent the sample being processed. But it doesn't explicitly say that the interpolation for Z and W work the same way, only that they are "interpolated". That is, it doesn't say where it is interpolated *to*, unlike X and Y.

Obviously it wouldn't make sense for X and Y to be interpolated to one of the samples but Z and W not to be. But it doesn't explicitly say.

Also, "depth value of the primitive" probably ought to be "depth value of the fragment".