Improve Float32/Float64 decoding performance by 64-65%; See azul3d/audio#5

Before
BenchmarkDecodeFloat32        50        487533093 ns/op
BenchmarkDecodeFloat64        50        525656683 ns/op

After
BenchmarkDecodeFloat32       100        175575936 ns/op
BenchmarkDecodeFloat64       100        181460060 ns/op