[gvar] fix infinite loop introduced by 11f3fca

The attempt on removing end_points had made the code unreadable
and has intrdouced infinite, fixed by making the code clear what
it tries to achieve.

src/hb-ot-var-gvar-table.hh

@@ -625,17 +625,27 @@ struct gvar
 	  deltas[pt_index].y += y_deltas[i] * scalar;
 	}
 
+	/* find point before phantoms start which is an end point */
+	unsigned all_contours_end = points.length ? points.length - 1 : 0;
+	while (all_contours_end > 0)
+	{
+	  if (points[all_contours_end].is_end_point) break;
+	  --all_contours_end;
+	}
+
 	/* infer deltas for unreferenced points */
-	for (unsigned start_point = 0; start_point + 4 < points.length; ++start_point)
+	for (unsigned start_point = 0; start_point < all_contours_end; ++start_point)
 	{
 	  /* Check the number of unreferenced points in a contour. If no unref points or no ref points, nothing to do. */
-	  unsigned unref_count = 0;
 	  unsigned end_point = start_point;
-	  do
+	  unsigned unref_count = 0;
+	  for (; end_point <= all_contours_end; ++end_point)
 	  {
-	    if (!deltas[end_point].flag) unref_count++;
-	    end_point++;
-	  } while (!points[end_point].is_end_point && end_point + 4 < points.length);
+	    if (!deltas[end_point].flag)
+	      unref_count++;
+	    if (points[end_point].is_end_point)
+	      break;
+	  }
 
 	  unsigned j = start_point;
 	  if (unref_count == 0 || unref_count > end_point - start_point)

test/api/test-ot-metrics-tt-var.c

@@ -234,6 +234,23 @@ test_advance_tt_var_comp_v (void)
   hb_font_destroy (font);
 }
 
+static void
+test_advance_tt_var_gvar_infer (void)
+{
+  hb_face_t *face = hb_test_open_font_file ("fonts/TestGVAREight.ttf");
+  hb_font_t *font = hb_font_create (face);
+  hb_ot_font_set_funcs (font);
+  hb_face_destroy (face);
+
+  int coords[6] = {100};
+  hb_font_set_var_coords_normalized (font, coords, 6);
+
+  hb_glyph_extents_t extents = {0};
+  g_assert (hb_font_get_glyph_extents (font, 4, &extents));
+
+  hb_font_destroy (font);
+}
+
 int
 main (int argc, char **argv)
 {
@@ -245,6 +262,7 @@ main (int argc, char **argv)
   hb_test_add (test_advance_tt_var_anchor);
   hb_test_add (test_extents_tt_var_comp);
   hb_test_add (test_advance_tt_var_comp_v);
+  hb_test_add (test_advance_tt_var_gvar_infer);
 
   return hb_test_run ();
 }