Expose height/depth info

Having written a bunch of "polyfills" for MathML, there were three pieces of missing functionality that required hacks to implement: find the height/depth of an element, convert a CSS length value to pixels (e.g., '3em'), and clone an element including it's shadow root. The later two issues are not specific to MathML, but MathML is somewhat unique in defining and using an element's height above the baseline and depth below the baseline.

Rather than having to rely on a hack, MathML elements should provide a method to get this information. This could be an extension of getBoundingClientRect that adds baseline info or depth (since height above baseline is easily computed from that) or it could be an entirely new method that returns height/depth/width info (the later often being useful when doing computations involving box dimensions.