LaTeX writer: allow specifying just width or height for image size.

Previously both needed to be specified (unless the image was
being resized to be smaller than its original size).

If height but not width is specified, we now set width to
textwidth (and similarly if width but not height is specified).
Since we have keepaspectratio, this yields the desired result.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -1124,7 +1124,12 @@ inlineToLaTeX (Image attr _ (source, _)) = do
                          Just dim         ->
                            [d <> text (show dim)]
                          Nothing          ->
-                           []
+                           case dir of
+                                Width | isJust (dimension Height attr) ->
+                                  [d <> "\\textwidth"]
+                                Height | isJust (dimension Width attr) ->
+                                  [d <> "\\textheight"]
+                                _ -> []
       dimList = showDim Width ++ showDim Height
       dims = if null dimList
                 then empty

test/command/3450.md

@@ -8,5 +8,5 @@
 % pandoc -fmarkdown-implicit_figures -t latex
 ![image](lalune.jpg){height=2em}
 ^D
-\includegraphics[height=2em]{lalune.jpg}
+\includegraphics[width=\textwidth,height=2em]{lalune.jpg}
 ```