[SVG output] mglyph alt creates (invalid) alt and title attributes

https://github.com/mathjax/MathJax/blob/419b0a6eee7eefc0f85e47f7d4f8227ec28b8e57/unpacked/jax/output/SVG/autoload/mglyph.js#L85

SVG does not allow alt and title attributes on image tags.

SVG2 and SVG in HTML5 allow aria-label however.