[varfonts] Map from OpenType to Fontconfig weight values

Oops.
diff --git a/src/fcfreetype.c b/src/fcfreetype.c
index 0d03208..cf1e196 100644
--- a/src/fcfreetype.c
+++ b/src/fcfreetype.c
@@ -1262,18 +1262,22 @@
 	      {
 		case FT_MAKE_TAG ('w','g','h','t'):
 		  elt = FC_WEIGHT;
+		  min_value = FcWeightFromOpenType (min_value);
+		  max_value = FcWeightFromOpenType (max_value);
 		  variable = variable_weight = FcTrue;
 		  weight = 0; /* To stop looking for weight. */
 		  break;
 
 		case FT_MAKE_TAG ('w','d','t','h'):
 		  elt = FC_WIDTH;
+		  /* Values in 'wdth' match Fontconfig FC_WIDTH_* scheme directly. */
 		  variable = variable_width = FcTrue;
 		  width = 0; /* To stop looking for width. */
 		  break;
 
 		case FT_MAKE_TAG ('o','p','s','z'):
 		  elt = FC_SIZE;
+		  /* Values in 'opsz' match Fontconfig FC_SIZE, both are in points. */
 		  variable = variable_size = FcTrue;
 		  break;
 	      }