Fix bug 2637

Fix bug 2637
        * src/graphics/GraphicsCacheItem.C
        (CacheItem::Impl::convertToDisplayFormat): Don't derive the temp
        file name from the original file name. This ensures that it is
        composed of valid characters only.

        * src/graphics/GraphicsConverter.C: remove example script, since it
        will get out of date.
        (build_script): Copy the original file to a temp file before the
        conversion chain starts. This avoids problems with converters that
        can't handle ' in filenames.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14459 a592a061-630c-0410-9148-cb99ea01b6c8