Add fbounds-safety annotations for VP8BitReader.

Reasoning:

Analysis of initialization (`VP8BitReaderSetBuffer`) and usage sites
(`src/utils/bit_reader_utils.c:59-61`,
`src/utils/bit_reader_inl_utils.h:87`,
`src/utils/bit_reader_utils.c:102`) showed:
- `buf` is the current read pointer, iterating towards `buf_end`.
- `buf_end` points one past the end of valid data (`start + size`).
- `buf_max` points to the last position for a safe read of `lbit_t`.
- All three pointers relate to the same buffer and are adjusted together
  in `VP8RemapBitReader`.

Annotations applied:
- `buf` is annotated `__ended_by(buf_end)` as it iterates up to
  `buf_end`.
- Self-assignments `br->buf_end = br->buf_end;` are added where
  `br->buf` is modified to satisfy the compiler check for `__ended_by`
  when the pointer is updated but the end bound remains the same.
- `buf_max` is annotated `__unsafe_indexable` because its arithmetic in
  `VP8RemapBitReader` couldn't be easily expressed with other
  annotations without ABI changes, and it's mainly adjusted during
  setup/remapping.
- The `start` parameters in `VP8BitReaderSetBuffer`
  (`src/utils/bit_reader_utils.h:116`,
  `src/utils/bit_reader_utils.c:34`) `VP8InitBitReader`
  (`src/utils/bit_reader_utils.h:113`, and
  `src/utils/bit_reader_utils.c:45`) are annotated `__counted_by(size)`
  because they receive a buffer start and size, and `start` using
  `size` within `VP8BitReaderSetBuffer`. This resolved errors
  arithmetic is performed on within the function and a subsequent
  warning when calling it from `VP8InitBitReader`.

Bug: 432511821
Change-Id: Ica436d9764b5bd1f7dcf1b54a280b6f60ecef4df

src/utils/bit_reader_inl_utils.h

@@ -85,6 +85,7 @@ static WEBP_UBSAN_IGNORE_UNDEF WEBP_INLINE void VP8LoadNewBytes(
     WEBP_UNSAFE_MEMCPY(&in_bits, br->buf, sizeof(in_bits));
 #endif
     br->buf += BITS >> 3;
+    WEBP_SELF_ASSIGN(br->buf_end);
 #if !defined(WORDS_BIGENDIAN)
 #if (BITS > 32)
     bits = BSwap64(in_bits);

src/utils/bit_reader_utils.c

@@ -31,7 +31,8 @@ WEBP_ASSUME_UNSAFE_INDEXABLE_ABI
 //------------------------------------------------------------------------------
 // VP8BitReader
 
-void VP8BitReaderSetBuffer(VP8BitReader* const br, const uint8_t* const start,
+void VP8BitReaderSetBuffer(VP8BitReader* const br,
+                           const uint8_t* const WEBP_COUNTED_BY(size) start,
                            size_t size) {
   assert(start != NULL);
   br->buf = start;
@@ -40,7 +41,8 @@ void VP8BitReaderSetBuffer(VP8BitReader* const br, const uint8_t* const start,
       (size >= sizeof(lbit_t)) ? start + size - sizeof(lbit_t) + 1 : start;
 }
 
-void VP8InitBitReader(VP8BitReader* const br, const uint8_t* const start,
+void VP8InitBitReader(VP8BitReader* const br,
+                      const uint8_t* const WEBP_COUNTED_BY(size) start,
                       size_t size) {
   assert(br != NULL);
   assert(start != NULL);
@@ -87,6 +89,7 @@ void VP8LoadFinalBytes(VP8BitReader* const br) {
   if (br->buf < br->buf_end) {
     br->bits += 8;
     br->value = (bit_t)(*br->buf++) | (br->value << 8);
+    WEBP_SELF_ASSIGN(br->buf_end);
   } else if (!br->eof) {
     br->value <<= 8;
     br->bits += 8;

src/utils/bit_reader_utils.h

@@ -103,17 +103,20 @@ struct VP8BitReader {
   range_t range;  // current range minus 1. In [127, 254] interval.
   int bits;       // number of valid bits left
   // read buffer
-  const uint8_t* buf;      // next byte to be read
+  const uint8_t* WEBP_ENDED_BY(buf_end) buf;  // next byte to be read
-  const uint8_t* buf_end;  // end of read buffer
+  const uint8_t* buf_end;                     // end of read buffer
-  const uint8_t* buf_max;  // max packed-read position on buffer
+  // max packed-read position on buffer
-  int eof;                 // true if input is exhausted
+  const uint8_t* WEBP_UNSAFE_INDEXABLE buf_max;
+  int eof;  // true if input is exhausted
 };
 
 // Initialize the bit reader and the boolean decoder.
-void VP8InitBitReader(VP8BitReader* const br, const uint8_t* const start,
+void VP8InitBitReader(VP8BitReader* const br,
+                      const uint8_t* const WEBP_COUNTED_BY(size) start,
                       size_t size);
 // Sets the working read buffer.
-void VP8BitReaderSetBuffer(VP8BitReader* const br, const uint8_t* const start,
+void VP8BitReaderSetBuffer(VP8BitReader* const br,
+                           const uint8_t* const WEBP_COUNTED_BY(size) start,
                            size_t size);
 
 // Update internal pointers to displace the byte buffer by the