Docx writer: support relative image widths.

Image widths given in percent are interpreted to be relative to the text
width. This is similar to the way percent widths are handled in other
writers, e.g. HTML and LaTeX. Previously, percent widths were taken
relative to the image's native size.

src/Text/Pandoc/Writers/Docx.hs

@@ -1345,8 +1345,10 @@ inlineToOpenXML' opts (Image attr@(imgident, _, _) alt (src, title)) = do
         (xpt,ypt) = desiredSizeInPoints opts attr
                (either (const def) id (imageSize opts img))
         -- 12700 emu = 1 pt
-        (xemu,yemu) = fitToPage (xpt * 12700, ypt * 12700)
-                                (pageWidth * 12700)
+        pageWidthPt = case dimension Width attr of
+                        Just (Percent a) -> pageWidth * (floor $ a * 127)
+                        _                -> pageWidth * 12700
+        (xemu,yemu) = fitToPage (xpt * 12700, ypt * 12700) pageWidthPt
         cNvPicPr = mknode "pic:cNvPicPr" [] $
                          mknode "a:picLocks" [("noChangeArrowheads","1")
                                              ,("noChangeAspect","1")] ()