Round the number of threads to use down to a power of 2

Since other calculations depend on this being true

geojson.c

@@ -70,6 +70,9 @@ void init_cpus() {
 		CPUS = 1;
 	}
 
+	// Round down to a power of 2
+	CPUS = 1 << (int) (log(CPUS) / log(2));
+
 	TEMP_FILES = 64;
 	struct rlimit rl;
 	if (getrlimit(RLIMIT_NOFILE, &rl) != 0) {