Improve gesture handling when current location tracking enabled

[sarahsnow1]

Allow pinching to occur. Currently, the map is re-zoomed if a new location is received when in the middle of a pinching gesture.

[msmollin]

After much exploration and testing, we're going to need a couple additions to Tangram to really enable a robust solution for this.
tangrams/tangram-es#1511
&
tangrams/tangram-es#1512

1511 is absolutely required to make this work correctly. 1512 is a "nice-to-have" because without it, during a pinch you can also get a shove or pan gesture recognition to happen and it will drag the zoom point off center. The next time a location update comes in though, it will recover and re-center itself, so it's not too bad and it does require some effort to make it happen.

[msmollin]

cc @karimnaaji / @matteblair / @tallytalwar

[msmollin]

For historical reference for implementation suggestions, see #125