BUG: FFMPEG linking fixes. Give ParaView access to VTK's FindFFMPEG. Don't fail if FIND can't find the library, instead give the user a chance to specify where it is. Don't automatically link against the library if it is found, because FFMPEG is likely to be too old to use. Give the user one variable to control whether it should be linked, and only show that if the lib is found.