use depth fog if not highp

[greggman]

just an idea, maybe @WestLangley has others?

[greggman]

#14736

[greggman]

mediump can't handle anything with large positions. All the specular lighting breaks in mediump too. See #14570 (comment)

[WestLangley]

mediump can't handle anything with large positions.

In the fiddle in #14736, the depth is 500. That is not big. Perhaps the problem is in squaring that to compute the length.

How about

float fogDepth = - vFogPosition.z * length( vFogPosition / vFogPosition.z );

Actually, you would have normalize by the max component. Consider this a proof-of-concept.

[mrdoob]

Closing in favour of #14752