Clarify comments in ImageOutput::copy_image
Clarify comments in ImageOutput::copy_image

Make it clear that this function is only copying the pixel data
from an ImageInput to an ImageOutput.

OpenImageIO/oiio,lgritz/oiio,lgritz/oiio,lgritz/oiio,OpenImageIO/oiio,OpenImageIO/oiio,lgritz/oiio,OpenImageIO/oiio