HTML writer: Better support for raw LaTeX environments.

Previously we just passed all raw TeX through when MathJax
was used for HTML math.  This passed through too much.
With this patch, only raw LaTeX environments that MathJax
can handle get passed through.

This patch also causes raw LaTeX environments to be treated
as math, when possible, with MathML and WebTeX output.

Closes #2758.

src/Text/Pandoc/Writers/HTML.hs

@@ -483,11 +483,9 @@ blockToHtml opts (Div attr@(ident, classes, kvs) bs) = do
         else addAttrs opts (ident, classes', kvs) $ divtag $ contents'
 blockToHtml opts (RawBlock f str)
   | f == Format "html" = return $ preEscapedString str
-  | f == Format "latex" =
-      case writerHTMLMathMethod opts of
-           MathJax _  -> do modify (\st -> st{ stMath = True })
-                            return $ toHtml str
-           _          -> return mempty
+  | (f == Format "latex" || f == Format "tex") &&
+     allowsMathEnvironments (writerHTMLMathMethod opts) &&
+     isMathEnvironment str = blockToHtml opts $ Plain [Math DisplayMath str]
   | otherwise          = return mempty
 blockToHtml opts (HorizontalRule) = return $ if writerHtml5 opts then H5.hr else H.hr
 blockToHtml opts (CodeBlock (id',classes,keyvals) rawCode) = do
@@ -811,13 +809,6 @@ inlineToHtml opts inline =
                          InlineMath  -> m
                          DisplayMath -> brtag >> m >> brtag 
     (RawInline f str)
-      | f == Format "latex" ->
-                          case writerHTMLMathMethod opts of
-                               LaTeXMathML _ -> do modify (\st -> st {stMath = True})
-                                                   return $ toHtml str
-                               MathJax _     -> do modify (\st -> st {stMath = True})
-                                                   return $ toHtml str
-                               _             -> return mempty
       | f == Format "html" -> return $ preEscapedString str
       | otherwise          -> return mempty
     (Link attr txt (s,_)) | "mailto:" `isPrefixOf` s -> do
@@ -915,3 +906,41 @@ renderKaTeX = unlines [
   , " katex.render(texText.data, mathElements[i])"
   , "}}"
   ]
+
+isMathEnvironment :: String -> Bool
+isMathEnvironment s = "\\begin{" `isPrefixOf` s &&
+                         envName `elem` mathmlenvs
+  where envName = takeWhile (/= '}') (drop 7 s)
+        mathmlenvs = [ "align"
+                     , "align*"
+                     , "alignat"
+                     , "alignat*"
+                     , "aligned"
+                     , "alignedat"
+                     , "array"
+                     , "Bmatrix"
+                     , "bmatrix"
+                     , "cases"
+                     , "CD"
+                     , "eqnarray"
+                     , "eqnarray*"
+                     , "equation"
+                     , "equation*"
+                     , "gather"
+                     , "gather*"
+                     , "gathered"
+                     , "matrix"
+                     , "multline"
+                     , "multline*"
+                     , "pmatrix"
+                     , "smallmatrix"
+                     , "split"
+                     , "subarray"
+                     , "Vmatrix"
+                     , "vmatrix" ]
+
+allowsMathEnvironments :: HTMLMathMethod -> Bool
+allowsMathEnvironments (MathJax _) = True
+allowsMathEnvironments (MathML _)  = True
+allowsMathEnvironments (WebTeX _)  = True
+allowsMathEnvironments _           = False