\color affects following \right, put array cells in their own groups (#1845)

* \color affects following \right

Fix #1844 by giving `leftright` nodes a `rightColor` attribute for how
to color the right bracket.  Now `\color` sets the macro
`\current@color` in the current environment (in particular, resetting
after `\right`), just like `color.sty` does in LaTeX.  This is used to
specially pass the current color into any following `\right` and then
into the `leftright` parse node.

* Add test

* Put each array cell in its own group/namespace

* Improve cell group isolation, add test and TODO

* Improve comments

src/functions/delimsizing.js

@@ -145,10 +145,16 @@ defineFunction({
         // \left case below triggers parsing of \right in
         //   `const right = parser.parseFunction();`
         // uses this return value.
+        const color = context.parser.gullet.macros.get("\\current@color");
+        if (color && typeof color !== "string") {
+            throw new ParseError(
+                "\\current@color set to non-string in \\right");
+        }
         return {
             type: "leftright-right",
             mode: context.parser.mode,
             delim: checkDelimiter(args[0], context).text,
+            color, // undefined if not set via \color
         };
     },
 });
@@ -178,6 +184,7 @@ defineFunction({
             body,
             left: delim.text,
             right: right.delim,
+            rightColor: right.color,
         };
     },
     htmlBuilder: (group, options) => {
@@ -241,12 +248,14 @@ defineFunction({
         }
 
         let rightDelim;
-        // Same for the right delimiter
+        // Same for the right delimiter, but using color specified by \color
         if (group.right === ".") {
             rightDelim = html.makeNullDelimiter(options, ["mclose"]);
         } else {
+            const colorOptions = group.rightColor ?
+                options.withColor(group.rightColor) : options;
             rightDelim = delimiter.leftRightDelim(
-                group.right, innerHeight, innerDepth, options,
+                group.right, innerHeight, innerDepth, colorOptions,
                 group.mode, ["mclose"]);
         }
         // Add it to the end of the expression.
@@ -273,6 +282,10 @@ defineFunction({
 
             rightNode.setAttribute("fence", "true");
 
+            if (group.rightColor) {
+                rightNode.setAttribute("mathcolor", group.rightColor);
+            }
+
             inner.push(rightNode);
         }