content: Correctly apply font-size to interpret "em" on the same KaTeX span

In CSS, the `em` unit is the font-size of the element, except when
defining font-size itself (in which case it's the font-size inherited
from the parent).  See MDN:
  https://developer.mozilla.org/en-US/docs/Web/CSS/length#em

So when the same HTML span has a declared value for font-size, and
also a declared value in units of `em` for some other property,
the declared font-size needs to be applied in order to correctly
interpret the meaning of `em` in the other property's value.

It's possible this never comes up in practice -- that KaTeX never
ends up giving us a span that gets both a font-size and a height.
If it were hard to correctly handle this, we might try to verify
that's the case and then rely on it (with an appropriate check to
throw an error if that assumption failed).  But the fix is easy,
so just fix it.

Author: gnprice

lib/widgets/content.dart

@@ -948,7 +948,7 @@ class _KatexSpan extends StatelessWidget {
 
     return SizedBox(
       height: styles.heightEm != null
-        ? styles.heightEm! * em
+        ? styles.heightEm! * (fontSize ?? em)
         : null,
       child: widget);
   }