Add fig-alt support to rgl graphs

[dmurdoch]

#336 correctly noted that the vignette had base graphics without alt text. However, all rgl plots also lack alt text, because the widget doesn't support it. That should be added.

It won't be trivial, because rgl figures are normally canvas elements, not img elements, and canvas doesn't support alt text. But there are workarounds, e.g. https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Basic_usage .

[dmurdoch]

I believe this is all done now, subject to knitr accepting my pull request: yihui/knitr#2243 . I'll leave the issue open until that is resolved.