backward_references_enc.h: fix WINDOW_SIZE_BITS check

this check was relocated in:
b903b80c Split cost-based backward references in its own file.

quiets -Wundef

Change-Id: I7f7a4773fb8cc77ca9f671b11f50d5db2275d415

src/enc/backward_references_enc.c

@@ -26,7 +26,6 @@
 #define MAX_ENTROPY    (1e30f)
 
 // 1M window (4M bytes) minus 120 special codes for short distances.
-#define WINDOW_SIZE_BITS 20
 #define WINDOW_SIZE ((1 << WINDOW_SIZE_BITS) - 120)
 
 // Minimum number of pixels for which it is cheaper to encode a

src/enc/backward_references_enc.h

@@ -116,6 +116,7 @@ static WEBP_INLINE uint32_t PixOrCopyDistance(const PixOrCopy* const p) {
 // If you change this, you need MAX_LENGTH_BITS + WINDOW_SIZE_BITS <= 32 as it
 // is used in VP8LHashChain.
 #define MAX_LENGTH_BITS 12
+#define WINDOW_SIZE_BITS 20
 // We want the max value to be attainable and stored in MAX_LENGTH_BITS bits.
 #define MAX_LENGTH ((1 << MAX_LENGTH_BITS) - 1)
 #if MAX_LENGTH_BITS + WINDOW_SIZE_BITS > 32