fix table-stream to support global ptrs

fontland/fontland/table-stream.rkt

@@ -39,11 +39,15 @@
 (define (decode-table this table-tag)
   (unless (hash-has-key? table-codecs table-tag)
     (raise-argument-error 'decode-table "decodable table" table-tag))
+
+  (define last-pos (pos (ttf-font-port this)))
+  (define stream (get-table-stream this table-tag))
+
+  (define table-decoder (hash-ref table-codecs table-tag))
   (define directory (force (ttf-font-directory this)))
   (define table (hash-ref (hash-ref directory 'tables) table-tag))
-  ;; todo: possible to avoid copying the bytes here?
-  (pos (ttf-font-port this) (hash-ref table 'offset))
-  (define table-bytes (open-input-bytes (peek-bytes (hash-ref table 'length) 0 (ttf-font-port this))))
-  (define table-decoder (hash-ref table-codecs table-tag))
-  (decode table-decoder table-bytes #:parent this))
+
+  (begin0
+    (decode table-decoder stream #:parent this (hash-ref table 'length))
+    (pos (ttf-font-port this) last-pos)))