Fix Cursive positioning

Test case: "مرا" rendered using IranNastaliq.

src/hb-ot-layout-gpos-private.hh

@@ -826,123 +826,6 @@ struct CursivePosFormat1
   inline bool apply (hb_apply_context_t *c) const
   {
     TRACE_APPLY ();
-    /* Now comes the messiest part of the whole OpenType
-       specification.  At first glance, cursive connections seem easy
-       to understand, but there are pitfalls!  The reason is that
-       the specs don't mention how to compute the advance values
-       resp. glyph offsets.  I was told it would be an omission, to
-       be fixed in the next OpenType version...  Again many thanks to
-       Andrei Burago <andreib@microsoft.com> for clarifications.
-
-       Consider the following example:
-
-			|  xadv1    |
-			 +---------+
-			 |         |
-		   +-----+--+ 1    |
-		   |     | .|      |
-		   |    0+--+------+
-		   |   2    |
-		   |        |
-		  0+--------+
-		  |  xadv2   |
-
-	 glyph1: advance width = 12
-		 anchor point = (3,1)
-
-	 glyph2: advance width = 11
-		 anchor point = (9,4)
-
-	 LSB is 1 for both glyphs (so the boxes drawn above are glyph
-	 bboxes).  Writing direction is R2L; `0' denotes the glyph's
-	 coordinate origin.
-
-       Now the surprising part: The advance width of the *left* glyph
-       (resp. of the *bottom* glyph) will be modified, no matter
-       whether the writing direction is L2R or R2L (resp. T2B or
-       B2T)!  This assymetry is caused by the fact that the glyph's
-       coordinate origin is always the lower left corner for all
-       writing directions.
-
-       Continuing the above example, we can compute the new
-       (horizontal) advance width of glyph2 as
-
-	 9 - 3 = 6  ,
-
-       and the new vertical offset of glyph2 as
-
-	 1 - 4 = -3  .
-
-
-       Vertical writing direction is far more complicated:
-
-       a) Assuming that we recompute the advance height of the lower glyph:
-
-				    --
-			 +---------+
-		--       |         |
-		   +-----+--+ 1    | yadv1
-		   |     | .|      |
-	     yadv2 |    0+--+------+        -- BSB1  --
-		   |   2    |       --      --        y_offset
-		   |        |
-     BSB2 --      0+--------+                        --
-	  --    --
-
-	 glyph1: advance height = 6
-		 anchor point = (3,1)
-
-	 glyph2: advance height = 7
-		 anchor point = (9,4)
-
-	 TSB is 1 for both glyphs; writing direction is T2B.
-
-
-	   BSB1     = yadv1 - (TSB1 + ymax1)
-	   BSB2     = yadv2 - (TSB2 + ymax2)
-	   y_offset = y2 - y1
-
-	 vertical advance width of glyph2
-	   = y_offset + BSB2 - BSB1
-	   = (y2 - y1) + (yadv2 - (TSB2 + ymax2)) - (yadv1 - (TSB1 + ymax1))
-	   = y2 - y1 + yadv2 - TSB2 - ymax2 - (yadv1 - TSB1 - ymax1)
-	   = y2 - y1 + yadv2 - TSB2 - ymax2 - yadv1 + TSB1 + ymax1
-
-
-       b) Assuming that we recompute the advance height of the upper glyph:
-
-				    --      --
-			 +---------+        -- TSB1
-	  --    --       |         |
-     TSB2 --       +-----+--+ 1    | yadv1   ymax1
-		   |     | .|      |
-	     yadv2 |    0+--+------+        --       --
-      ymax2        |   2    |       --                y_offset
-		   |        |
-	  --      0+--------+                        --
-		--
-
-	 glyph1: advance height = 6
-		 anchor point = (3,1)
-
-	 glyph2: advance height = 7
-		 anchor point = (9,4)
-
-	 TSB is 1 for both glyphs; writing direction is T2B.
-
-	 y_offset = y2 - y1
-
-	 vertical advance width of glyph2
-	   = TSB1 + ymax1 + y_offset - (TSB2 + ymax2)
-	   = TSB1 + ymax1 + y2 - y1 - TSB2 - ymax2
-
-
-       Comparing a) with b) shows that b) is easier to compute.  I'll wait
-       for a reply from Andrei to see what should really be implemented...
-
-       Since horizontal advance widths or vertical advance heights
-       can be used alone but not together, no ambiguity occurs.        */
-
     struct hb_ot_layout_context_t::info_t::gpos_t *gpi = &c->layout->info.gpos;
     hb_codepoint_t last_pos = gpi->last;
     gpi->last = HB_OT_LAYOUT_GPOS_NO_LAST;
@@ -965,15 +848,14 @@ struct CursivePosFormat1
 
     /* TODO vertical */
 
+    /* Align the exit anchor of the left glyph with the entry anchor of the right glyph. */
     if (c->buffer->props.direction == HB_DIRECTION_RTL)
     {
-      /* advance is absolute, not relative */
-      c->buffer->pos[c->buffer->i].x_advance = entry_x - gpi->anchor_x;
+      c->buffer->pos[c->buffer->i].x_advance = c->buffer->pos[c->buffer->i].x_offset + entry_x - gpi->anchor_x;
     }
     else
     {
-      /* advance is absolute, not relative */
-      c->buffer->pos[last_pos].x_advance = gpi->anchor_x - entry_x;
+      c->buffer->pos[last_pos].x_advance = c->buffer->pos[last_pos].x_advance + gpi->anchor_x - entry_x;
     }
 
     if  (c->lookup_flag & LookupFlag::RightToLeft)