IOMemReader ctr should take const buffer pointer (#3665)
IOMemReader ctr should take const buffer pointer (#3665)

The ctr is inline, so I don't think it breaks backwards link
compatibility to make this change, so I do intend to backport it.
OpenImageIO/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,lgritz/oiio