Rationale

Due to the design of PAR_REALLOC it can't work with some kinds of allocators that are not designed to support realloc. A good way to solve this, as seen in other libraries such as stb, is to have the PAR_REALLOC macro also provide the old size of the buffer. I specifically had this issue with par_shapes so I only provided a solution for that header.

Changes

I changed the PAR_REALLOC macro to accept an extra parameter OLD_SZ that represents the old size.
The default implementation of the macro remains unchanged.
I then searched for all the uses of PAR_REALLOC in par_shapes and added the old size of the buffers to the PAR_REALLOC call. This sometimes involved creating new variables such as int old_dst_npoints = dst->npoints;.

Testing

I tested my solution by adding a small realloc wrapper in test_shapes.c which uses malloc and free and overriding the PAR_* allocation macros.

All tests have passed when I ran them after I applied my changes.

Considerations

This will affect people who already override PAR_REALLOC since they will have to add the extra parameter to the macro, however, if their PAR_REALLOC implementation already works without needing the old size then they can just ignore the OLD_SZ parameter, this is what the default implementation for PAR_REALLOC does too.

This will cause an inconsistency in the API however since all other par headers will still have the old-style realloc. If needed I can provide this change for the other par headers as well.