It is not possible to start a svg-graphic inside a fo:block at coordinate x=0, 
y=0.
there is always a small offset in y-direction (of about 1px).

here is an example. When zooming into the rendered pdf with Acrobat,
you will see, that the black triangle does not start where the block 
with red background color starts.


<?xml version="1.0" encoding="ISO-8859-1"?>
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <fo:layout-master-set>
     <fo:simple-page-master master-name="one_column" page-height="11.488mm" 
page-width="45.0mm" margin-top="0.0mm" margin-bottom="0.0mm" margin-
left="0.0mm" margin-right="0.0mm">
      <fo:region-body/>
    </fo:simple-page-master>
  </fo:layout-master-set>
  <fo:page-sequence master-reference="one_column">
    <fo:flow flow-name="xsl-region-body">
      <fo:block background-color="red"> 
 		<fo:instream-foreign-object>
		<svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="11.488mm" 
height="11.488mm">
		  <svg:title>triangle</svg:title>
   			<svg:g style="fill: #000000; stroke:#000000; stroke-
width:0.172">
			<svg:path d="M 0 0 L 16 16 L 0 32 z"/>
  			</svg:g> 
 		</svg:svg>
		</fo:instream-foreign-object>
    </fo:block>
     </fo:flow>
  </fo:page-sequence>
</fo:root>