Polyfill updates

Based on the resolutions from Monday's meeting, I updated the mpadded
polyfill so that handles all pseudo unit attr values.

I also created a simple polyfill to handle mglyph element. I only handled
the MathML 3 mglyph attrs, not the deprecated ones. Two wrinkles:
1. In MathML, width/height are length attr; for image, they are unitless
values that specify the image size in pixels. That means that the polyfill
has to play games to interpret 'em's etc.

2. valgin maps to CSS's 'vertical-align' property.  This works in Chrome
Canary. In Firefox, this had no effect. I think a WPT test needs to be
added to catch/test (and hopefully get fixed) this incompatibility.

You can try out at
https://mathml-refresh.github.io/mathml-polyfills/acid-test.html

<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail>
Virus-free.
www.avg.com
<http://www.avg.com/email-signature?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=webmail>
<#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>