The clustering behavior is strange when the zoom level is above 20

[strstrst]

Supercluster stores all positions internally on a 32-bit floating point array. When it converts these positions to 32 bits with Math.fround, 2 different positions can be converted to the same position. As a result, these 2 positions are always clustered. Could you add a parameter to configure the use of Math.fround?

[andrewharvey]

See #244