oes_texture_float not well-supported on phones

[mk30]

remove use of oes_texture_float in picking.

[ralphtheninja]

FYI this only seems to be a problem on mobile chrome but works fine on mobile firefox (android). Also, picking might work differently on mobile? E.g. clicking the mouse on desktop and panning the map might need some re-work on mobile, since you need the gesture events to pan the map. Maybe this should be a separate issue, not sure.