Fix projection pushdown with offset encodings

[jonmmease]

Fix projection pushdown to detect column usage inside the offset encoding inside a parent position encoding. For example:

          "x": {
            "value": 0,
            "offset": {
              "scale": "x", "field": "Horsepower"
            }
          }

I don't see documentation for this in https://vega.github.io/vega-lite/docs/encoding.html#position, but Vega-Lite produces it sometimes.