Preserve order of filters when removing

Filters should be kept in order since the first one that matches,
wins. This lets highlights or ignores take priority over each other,
if desired.

filter.c

@@ -74,7 +74,7 @@ bool filterRemove(struct Filter filter) {
 		if (filter.chan && strcasecmp(filters[i].chan, filter.chan)) continue;
 		if (filter.mesg && strcasecmp(filters[i].mesg, filter.mesg)) continue;
 		free(filters[i].mask);
-		filters[i] = filters[--len];
+		memmove(&filters[i], &filters[i + 1], sizeof(*filters) * --len);
 		filters[len] = (struct Filter) {0};
 		found = true;
 	}