Refactor palette functions to write a general color map

This will allow the mrc-tx experiment to pass in its own color map
and set of probabilities to the tokenize and bit packing functions.
The corresponding change on the decoder side will come in a followup
shortly.

No change in performance.

Change-Id: I1872b7f1b1cf5b102e6289388ce1ae449e67cb4d
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 1582250..0c211b1 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -624,7 +624,7 @@
   ++p;
   --num;
   for (int i = 0; i < num; ++i) {
-    aom_write_symbol(w, p->token, p->palette_cdf, n);
+    aom_write_symbol(w, p->token, p->color_map_cdf, n);
     ++p;
   }
   *tp = p;