[XeTeX] Asymptote support xetex engine issue.

[yunhual]

Both graphics include parts with negative coordinates. ,,,

Yes. you are right, it is caused by negative coordinate. I have checked
the source of xetex. xelatex engine first transfer tex file to xdv file.
then the xdvipdfmx will transfer the xdv file to pdf. actually the xdv
file consist some binary data and include some EPS file(if it include
some EPS picture like our previous example). for example the EPS file of
the circle. In the internal, xdvipdfmx will call the gs(ghostscript)
utility  to transfer each EPS file to pdf file. and then build the final
output pdf file acoording to the each picture(pdf from eps) and the
binary data in the xdv file. 
because of the negative coordinate,  correspond second, third and
fourth quadrants of the circle not show. but we can use the -dEPSCrop
paramater to let gs adjust correspond of the negative coordinate. the
default command line to the gs is saved in the  dvipdfmx.cfg file.  But
only the coordinate of the EPS have adjust by the gs. if some label in
xdv file. we got wrong output. for example, the asy file like this:

size(0,3cm);
draw(unitcircle,red);
label("C",(0,0));

run asy -k test.asy.   
-k will keep all keep all intemediate files. so will got test_0.eps. it
just contian the full circle. and a tex file. it include test_0.eps and
the label of "C". if I pass  -dEPSCrop option to gs utility through the
dvipdfmx.cfg. I can get a full circle, but the coordinate of the label
"C" is wrong. so in the final pdf file, the label "C" is not in the
center of the circle. 

thank you very much.