Use exr header to ensure output is tiled rather than m_spec.
Use exr header to ensure output is tiled rather than m_spec.

In the case of multi-part exrs, m_spec will not yet have been
initialized when exr headers are being created from incoming specs.

lgritz/oiio,OpenImageIO/oiio,mcanthony/oiio,mcanthony/oiio,cwilling/oiio,OpenImageIO/oiio,sambler/oiio,micler/oiio,micler/oiio,sambler/oiio,YangYangTL/oiio,micler/oiio,scott-wilson/oiio,scott-wilson/oiio,lgritz/oiio,mcanthony/oiio,sambler/oiio,cwilling/oiio,YangYangTL/oiio,mcanthony/oiio,lgritz/oiio,OpenImageIO/oiio,bdeluca/oiio,bdeluca/oiio,cwilling/oiio,scott-wilson/oiio,OpenImageIO/oiio,bdeluca/oiio,cwilling/oiio,bdeluca/oiio,YangYangTL/oiio,micler/oiio,sambler/oiio,scott-wilson/oiio,lgritz/oiio,YangYangTL/oiio