ketShaderPermute = (head, body, span=null) => ketShader(
    head + `float _ketgen_input_for(float out_id) { ${body} }`,
    'return inp(_ketgen_input_for(out_id));',
    span)
@param {!String} head
@param {!String} body
@param {null|!int=null} span
@return {!{withArgs: !function(args: ...!WglArg|!WglTexture) : !WglConfiguredShader}}