add viewBox to inline SVGs

[xworld21]

Inline SVGs generated by LaTeXML do not have a @viewBox attribute, which makes it virtually impossible to resize them from CSS (e.g. setting max-width: 100% to ensure that the image fits cannot work without @viewBox).

[xworld21]

After some digging, @viewBox was intentionally removed in #1657, although I do not understand why.

For instance here

LaTeXML/lib/LaTeXML/Package/pgfsys-latexml.def.ltxml

Line 90 in 40e9fa8

#### viewBox => "$props{minx} $props{miny} $props{pxwidth} $props{pxheight}",

uncommenting the line seems ok (in fact, minx and miny are always 0, so many of those calculations are superfluous).

I might send a PR restoring some viewBox here and there. The one for xypic is more challenging, as it looks like you are using the SVG transform to mimic vertical-align (badly, because you still get the wrong CSS box), so it's not entirely obvious what the correct viewBox should be.