Found another copy in renderer/Cinematic.cpp. Move the
newer implementation from renderer/Image_files.cpp in its
own file and use the libjpeg v8c license blob (which is where
this code comes from).
Adapt README too with the v8c license.
This function only exists starting with libjpeg8.
Check for existence and provide an implementation if necessary.
This allows libjpeg6 to be used.
Code borrowed from libjpeg8, adapt copyright in README
accordingly.
