Properly compute token buffer size

This accounts for border extension necessary when image size is not
a multiple of SB size in either horizontal or vertical direction.

BUG=aomedia:647

Change-Id: I18ae9edf148a83f8b32a06ca31410e30ebaa5a47

av1/encoder/encoder.h

@@ -705,10 +705,10 @@ static INLINE int enc_is_ref_frame_buf(AV1_COMP *cpi, RefCntBuffer *frame_buf) {
 static INLINE unsigned int get_token_alloc(int mb_rows, int mb_cols) {
   // We assume 3 planes all at full resolution. We assume up to 1 token per
   // pixel, and then allow a head room of 1 EOSB token per 4x4 block per plane,
-  // plus EOSB_TOKEN per plane, plus a 8 token margin.
-  // TODO(sebastien): Figure out the exact max size needed.
-  // See: https://crbug.com/aomedia/647
-  return mb_rows * mb_cols * (16 * 16 + 17 + 8) * 3;
+  // plus EOSB_TOKEN per plane.
+  // ALIGN_POWER_OF_TWO is used to round image size up to multipel of SBs
+  return ALIGN_POWER_OF_TWO(mb_rows, MAX_SB_SIZE_LOG2 - 4) *
+         ALIGN_POWER_OF_TWO(mb_cols, MAX_SB_SIZE_LOG2 - 4) * (16 * 16 + 17) * 3;
 }
 
 // Get the allocated token size for a tile. It does the same calculation as in