fix a critical precision issue

lib/fimg-math.c

@@ -47,21 +47,24 @@ return maxval;
 int fimg_meanvalues(FloatImg *head, float means[4])
 {
 int		idx, surface;
+double		accus[4];
 
 surface = head->width * head->height;
 if (surface < 1)	return -1;
 
-memset(means, 0, 4*sizeof(float));
+memset(accus, 0, 4*sizeof(double));
 
 for (idx=0; idx<surface; idx++) {
-	means[0] += head->R[idx];
+	accus[0] += head->R[idx];
 	if (head->type > 2) {
-		means[1] += head->G[idx];
-		means[2] += head->B[idx];
+		accus[1] += head->G[idx];
+		accus[2] += head->B[idx];
 		}
 	}
 
-for (idx=0; idx<4; idx++)	means[idx] /= (float)surface;
+for (idx=0; idx<4; idx++) {
+	means[idx] = (float)(accus[idx]/(double)surface);
+	}
 
 return 0;
 }