Fix small-radius clusters not always forming on higher zooms

[mourner]

Fixes mapbox/mapbox-gl-js#2898. Turned out to be a very tricky bug!

Basically, if you set a small cluster radius, same-location points would sometimes fail to cluster on higher zooms, because the point KD-tree index for querying uses Float32Array internally for smaller memory footprint, and that's generally enough precision for location data, except in case where we do small radius queries — in this case the query point was float64, while stored coordinates were float32, leading to the query returning zero results.

The fix is to round the internal stored coordinates to float32 outside the index as well, so that the values are consistent. Math.fround is the fast way to do that, but I'm still including a fallback for IE11 so that non-GL-JS consumers could benefit from the fix.

Doesn't need C++ port because supercluster.hpp uses double precision for the index.