Fix #120579: incorrect compute context hashes

The problem was that `XXH3_128bits` was called on `len` bytes
and not `HashSizeInBytes + len` as before 51f8bf53b2.
This lead to more compute context duplicates that one would expect.

I changed the code a little bit to make this mistake less likely in case
the hash function is ever changed to something else.

source/blender/blenlib/BLI_compute_context.hh

@@ -47,7 +47,6 @@ namespace blender {
  * have enough bits to make collisions practically impossible.
  */
 struct ComputeContextHash {
-  static constexpr int64_t HashSizeInBytes = 16;
   uint64_t v1 = 0;
   uint64_t v2 = 0;
 
@@ -63,8 +62,6 @@ struct ComputeContextHash {
   friend std::ostream &operator<<(std::ostream &stream, const ComputeContextHash &hash);
 };
 
-static_assert(sizeof(ComputeContextHash) == ComputeContextHash::HashSizeInBytes);
-
 /**
  * Identifies the context in which a computation happens. This context can be used to identify
  * values logged during the computation. For more details, see the comment at the top of the file.

source/blender/blenlib/intern/compute_context.cc

@@ -11,12 +11,14 @@ namespace blender {
 
 void ComputeContextHash::mix_in(const void *data, int64_t len)
 {
-  DynamicStackBuffer<> buffer_owner(HashSizeInBytes + len, 8);
+  const int64_t hash_size = sizeof(ComputeContextHash);
+  const int64_t buffer_len = hash_size + len;
+  DynamicStackBuffer<> buffer_owner(buffer_len, 8);
   char *buffer = static_cast<char *>(buffer_owner.buffer());
-  memcpy(buffer, this, HashSizeInBytes);
-  memcpy(buffer + HashSizeInBytes, data, len);
+  memcpy(buffer, this, hash_size);
+  memcpy(buffer + hash_size, data, len);
 
-  const XXH128_hash_t hash = XXH3_128bits(buffer, len);
+  const XXH128_hash_t hash = XXH3_128bits(buffer, buffer_len);
   memcpy(this, &hash, sizeof(hash));
   static_assert(sizeof(ComputeContextHash) == sizeof(hash));
 }