Clean up the gamma semantics a little.

0 (default) means unchanged: maxzoom contains all the dots.

Positive number, even very small, means that it still checks
the gap size and thins out dots if they are close together.

Negative number means no gamma, and the old random instead of
uniform dropping at low zooms.

geojson.c

@@ -901,7 +901,7 @@ int main(int argc, char **argv) {
 	int minzoom = 0;
 	int force = 0;
 	double droprate = 2.5;
-	double gamma = 1;
+	double gamma = 0;
 	int buffer = 5;
 	const char *tmpdir = "/tmp";