Misc Changes (Described further)

1. Added debug flags to ios cmake
2. pan velocity takes zoom level into account now, fixes the inconsistent pan speed when at different zoom levels.
3. added some checks in View::setZoom to better handle valid tile IDs
Merge branch 'master' of https://github.com/tangram-map/tangram-es
Merge branch 'master' of https://github.com/tangram-map/tangram-es