PDF: with xelatex, don't compress images til the last run.

This saves time for image-heavy documents.  Closes #4484.

src/Text/Pandoc/PDF.hs

@@ -277,7 +277,12 @@ runTeXProgram verbosity program args runNumber numRuns tmpDir source = do
     let file' = file
 #endif
     let programArgs = ["-halt-on-error", "-interaction", "nonstopmode",
-         "-output-directory", tmpDir'] ++ args ++ [file']
+         "-output-directory", tmpDir'] ++
+         -- see #4484, only compress images on last run:
+         if program == "xelatex" && runNumber < numRuns
+            then ["-output-driver", "xdvipdfmx -z0"]
+            else []
+         ++ args ++ [file']
     env' <- getEnvironment
     let sep = [searchPathSeparator]
     let texinputs = maybe (tmpDir' ++ sep) ((tmpDir' ++ sep) ++)