Consolidating logic to set the "center of action" for touch controls
Consolidating logic to set the "center of action" for touch controls

It's now always set when a new touch sequence is started, instead of
only for zoom and azimuth rotation gestures. Although it's still only
used for those two gestures, this will enable it to be used for the
panning implementation as well (in a future commit).

jstrait/webgl-city,jstrait/webgl-city