Fluid Particles: fix threading crash with viscoelastic springs.

As correctly pointed out by a comment in the code, adding
new springs wasn't thread safe, and caused crashes.

Fix by buffering new springs in intermediate thread-local
arrays, which are flushed on the main thread. This is valid
because the new springs are not used until the next sim step.

Differential Revision: https://developer.blender.org/D6133

source/blender/blenlib/BLI_buffer.h

@@ -83,4 +83,14 @@ void _bli_buffer_free(BLI_Buffer *buffer);
   } \
   (void)0
 
+/* A buffer embedded in a struct. Using memcpy is allowed until first resize. */
+#define BLI_buffer_field_init(name_, type_) \
+  { \
+    memset(name_, 0, sizeof(*name_)); \
+    *(size_t *)&((name_)->elem_size) = sizeof(type_); \
+  } \
+  (void)0
+
+#define BLI_buffer_field_free(name_) _bli_buffer_free(name_)
+
 #endif /* __BLI_BUFFER_H__ */