default.latex: Redefine includegraphics to allow custom options.

Thanks to Bernhard Weichel.
Now, literal \includegraphics commands with square-bracketed options
will work properly.  When no options are given, a default option
limits images to the text width.

default.latex

@@ -65,15 +65,19 @@ $if(tables)$
 $endif$
 $if(graphics)$
 \usepackage{graphicx}
-% We will generate all images so they have a width \maxwidth. This means
-% that they will get their normal width if they fit onto the page, but
+% Redefine \includegraphics so that, unless explicit options are
+% given, the image width will not exceed the width of the page.
+% Images get their normal width if they fit onto the page, but
 % are scaled down if they would overflow the margins.
 \makeatletter
 \def\maxwidth{\ifdim\Gin@nat@width>\linewidth\linewidth
 \else\Gin@nat@width\fi}
 \makeatother
 \let\Oldincludegraphics\includegraphics
-\renewcommand{\includegraphics}[1]{\Oldincludegraphics[width=\maxwidth]{#1}}
+{%
+ \catcode`\@=11\relax%
+ \gdef\includegraphics{\@ifnextchar[{\Oldincludegraphics}{\Oldincludegraphics[width=\maxwidth]}}%
+}%
 $endif$
 \ifxetex
   \usepackage[setpagesize=false, % page size defined by xetex