r126: filter by fraction of seed coverage

otherwise we may get too many poor overlap mappings.

minimap.h

@@ -75,6 +75,7 @@ typedef struct {
 	int max_chain_skip;
 	int min_cnt;
 	int min_chain_score;
+	float min_seedcov_ratio;
 
 	float pri_ratio;
 	float mask_level;