add Main-BoldItalic font to allow nesting of \textit and \textbf

test/screenshotter/ss_data.yaml

@@ -296,6 +296,7 @@ TextStacked:
       \text{abc123 \textbf{abc123} \textit{abc123}}\\
       \textrm{abc123 \textbf{abc123} \textit{abc123}}\\
       \textsf{\textrm{\textbf{abc123}} \textbf{abc123} \textit{abc123}}\\
+      \textit{abc123 \textbf{abc123} \textsf{abc123}}\\
   \end{matrix}
 TextWithMath: \text{for $a < b$ and $ c < d $}.
 Unicode: \begin{matrix}\text{ÀàÇçÉéÏïÖöÛû} \\ \text{БГДЖЗЙЛФЦШЫЮЯ} \\ \text{여보세요} \\ \text{私はバナナです} \end{matrix}