Frame buffer slice strides are non-negative as per:
https://github.com/openexr/openexr/blob/d778a0b47b18fe1ede3824fe013cea9dd7404360/OpenEXR/IlmImf/ImfFrameBuffer.h#L98-L99

Negative strides could be useful for flipping images at I/O time.

Would it be possible to change size_t to ssize_t at the next ABI-break-point?