Add fbounds-safety annotations for VP8BitWriter.

Reasoning:

The `fbounds-safety` compiler extension reported out-of-bounds accesses
on the `buf` member of the `VP8BitWriter` struct (defined in
`src/utils/bit_writer_utils.h`, line 36). These occurred in
`src/utils/bit_writer_utils.c` at lines 70, 74, 76, and 189, where
`buf` was used with array indexing or pointer arithmetic despite being a
`__single` pointer by default.

To fix this, the `buf` member was annotated as
`__sized_by_or_null(max_pos)` in `src/utils/bit_writer_utils.h`
(line 36), associating it with the `max_pos` member which stores the
buffer size.

This annotation introduced a new build error in the `BitWriterResize`
function (`src/utils/bit_writer_utils.c`, line 55) when assigning
the result of `WebPSafeMalloc` (an `__unsafe_indexable` pointer) to the
now-annotated `bw->buf`. This was resolved by:
1. Using `bw->buf = __unsafe_forge_bidi_indexable(uint8_t*, new_buf,
   new_size);` (line 55) to create a properly bounded pointer from the
  `malloc` result (`new_buf`) using its size (`new_size`) before
  assigning it to `bw->buf`.

Bug: 432511821
Change-Id: I1a24a9a432388ccce53a5e9b3b5e58d22aa61d0c

src/utils/bit_writer_utils.c

@@ -52,7 +52,7 @@ static int BitWriterResize(VP8BitWriter* const bw, size_t extra_size) {
     WEBP_UNSAFE_MEMCPY(new_buf, bw->buf, bw->pos);
   }
   WebPSafeFree(bw->buf);
-  bw->buf = new_buf;
+  bw->buf = WEBP_UNSAFE_FORGE_BIDI_INDEXABLE(uint8_t*, new_buf, new_size);
   bw->max_pos = new_size;
   return 1;
 }

src/utils/bit_writer_utils.h

@@ -34,7 +34,8 @@ struct VP8BitWriter {
   int32_t value;
   int run;      // number of outstanding bits
   int nb_bits;  // number of pending bits
-  uint8_t* buf;  // internal buffer. Re-allocated regularly. Not owned.
+  // internal buffer. Re-allocated regularly. Not owned.
+  uint8_t* WEBP_SIZED_BY_OR_NULL(max_pos) buf;
   size_t pos;
   size_t max_pos;
   int error;  // true in case of error