Fix bug 2637

Fix bug 2637
	* src/graphics/GraphicsConverter.C
	(Converter::Impl::Impl): Don't call the default converter directly,
	but create a temporary script with build_script() as for the
	configured converters. This makes sure that the file name does not
	need to be passed on the command line anymore.
	(build_script): This cannot fail anymore, so change the return type
	to void
	(build_script): Use build_conversion_command also for the default
	converter. This has the advantage that the special code for moving
	${outfile}.0, ${outfile}.1 is actually used for ImageMagick's convert.
	(build_conversion_command): factored out from build_script

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14657 a592a061-630c-0410-9148-cb99ea01b6c8