r708: implemented double Z-drop thresholds (#112)

When aligning long reads, we would prefer to align through low-quality
regions. This requires a large Z-drop threshold. However, to find small
inversions, we need to use a small Z-drop. This commit address this
conflict with two Z-drop thresholds. When Z-drop exceeds the smaller
threshold, we perform a local alignment to check if there is a potential
inversion. If there is one, we break the alignment; otherwise we break
the alignment only if Z-drop excess the larger threshold.

This commit also fixes a bug that reported wrong coordinates when the
inversion is on the forward strand (#112).

hit.c

@@ -94,6 +94,7 @@ void mm_split_reg(mm_reg1_t *r, mm_reg1_t *r2, int n, int qlen, mm128_t *a)
 	r2->id = -1;
 	r2->sam_pri = 0;
 	r2->p = 0;
+	r2->split_inv = 0;
 	r2->cnt = r->cnt - n;
 	r2->score = (int32_t)(r->score * ((float)r2->cnt / r->cnt) + .499);
 	r2->as = r->as + n;