composetable: Accept short compose sequences

Followup to 8931169e00. That commit did
not do enough work to actually accept sequences
of lenth 1, as pointed by Mike Fabian.

gtk/gtkcomposetable.c

@@ -1331,11 +1331,28 @@ gtk_compose_table_check (const GtkComposeTable *table,
   if (!seq_index)
     return FALSE;
 
-  if (n_compose == 1)
-    return TRUE;
-
   match = FALSE;
 
+  if (n_compose == 1)
+    {
+      if (seq_index[2] - seq_index[1] > 0)
+        {
+          seq = table->data + seq_index[1];
+
+          value = seq[0];
+
+          if ((value & (1 << 15)) != 0)
+            g_string_append (output, &table->char_data[value & ~(1 << 15)]);
+          else
+            g_string_append_unichar (output, value);
+
+          if (compose_match)
+            *compose_match = TRUE;
+        }
+
+      return TRUE;
+    }
+
   for (i = n_compose - 1; i < table->max_seq_len; i++)
     {
       len = i + 1;

gtk/gtkcomposetable.h

@@ -30,12 +30,13 @@ typedef struct _GtkComposeTableCompact GtkComposeTableCompact;
  * The first part of the data contains rows of length max_seq_len + 1,
  * where the first element is the item of the sequence, and the
  * following elements are offsets to the data for sequences that
- * start with the first item of length 2, ..., max_seq_len.
+ * start with the first item of length 1, ..., max_seq_len.
  *
  * The second part of the data contains the rest of the sequence
  * data. It does not have a fixed stride. For each sequence, we
  * put seq[2], ..., seq[len - 1], followed by the encoded value
- * for this sequence.
+ * for this sequence. In particular for a sequence of length 1,
+ * the offset points directly to the value.
  *
  * The values are encoded as follows:
  *