Fix \mathit font and italic correction and add \mathnormal (#1700)

* Fix \mathit font (Math-Italic -> Main-Italic) and italic correction

* Rename mathit to mathdefault and mainit to mathit

* Add \mathnormal

test/screenshotter/ss_data.yaml

@@ -200,6 +200,7 @@ MathBf: \mathbf{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathCal: \mathcal{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathFrak: \mathfrak{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathIt: \mathit{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
+MathNormal: \mathnormal{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathOp: a\mathop+b\mathop:c\mathop{\delta}e\mathop{\textrm{and}}f\mathrel{\mathop{:}}=g\sin h
 MathRm: \mathrm{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}
 MathSf: \mathsf{Ax2k\breve{a}\omega\Omega\imath+\KaTeX}