Fix \underset (#1277)

* Fix \underset

This PR modifies `\underset` and `\overset` to align with LaTeX behavior when a superscript is applied after the `\underset`.  Fixes issue #1275.

I also changed the spacing from KaTeX `mop` to KaTeX `rmel`. The result seems to be a closer visual match to LaTeX.

* Update screenshots

test/katex-spec.js

@@ -2779,11 +2779,9 @@ describe("A macro expander", function() {
     });
 */
 
-    it("should expand the \\overset macro as expected", function() {
-        expect("\\overset?=").toParseLike("\\mathop{=}\\limits^{?}");
-        expect("\\overset{x=y}{\\sqrt{ab}}")
-            .toParseLike("\\mathop{\\sqrt{ab}}\\limits^{x=y}");
-        expect("\\overset {?} =").toParseLike("\\mathop{=}\\limits^{?}");
+    it("should build \\overset and \\underset", function() {
+        expect("\\overset{f}{\\rightarrow} Y").toBuild();
+        expect("\\underset{f}{\\rightarrow} Y").toBuild();
     });
 
     it("should build \\iff, \\implies, \\impliedby", function() {

test/screenshotter/ss_data.yaml

@@ -218,7 +218,7 @@ OpLimits: |
 OverUnderline: x\underline{x}\underline{\underline{x}}\underline{x_{x_{x_x}}}\underline{x^{x^{x^x}}}\overline{x}\overline{x}\overline{x^{x^{x^x}}} \blue{\overline{\underline{x}}\underline{\overline{x}}}
 OverUnderset: |
     \begin{array}{l}
-    x\overset?=1\\
+    x\overset?=1 \quad \underset{*}{x}^2 \\
     {\displaystyle\lim_{t\underset{>0}\to0}}\\
     a+b+c+d\overset{b+c=0}\longrightarrow a+d\\
     \overset { x = y } { \sqrt { a b } }