Support {rcases} and {drcases} (#2149)

* Support {rcases} and {drcases}

* Fix lint error

* Update screenshots

test/katex-spec.js

@@ -2646,6 +2646,15 @@ describe("A cases environment", function() {
 
 });
 
+describe("An rcases environment", function() {
+
+    it("should build", function() {
+        expect`\begin{rcases} a &\text{if } b \\ c &\text{if } d \end{rcases}⇒…`
+            .toBuild();
+    });
+
+});
+
 describe("An aligned environment", function() {
 
     it("should parse its input", function() {

test/screenshotter/ss_data.yaml

@@ -74,7 +74,11 @@ Cases: |
         a+1&\text{if }b\text{ is odd} \\
         a&\text{if }b=0 \\
         a-1&\text{otherwise}
-    \end{cases}
+    \end{cases} \\
+    \begin{rcases}
+        a &\text{if } b \\
+        c &\text{if } d
+    \end{rcases}⇒…
 Colors:
     tex: \blue{a}\textcolor{#0f0}{b}\textcolor{red}{c}
     nolatex: different syntax and different scope