rgl 1.2.8

Minor changes

• ARIA support now declares rgl scenes with role = "img".

Bug fixes

• The ARIA support caused htmlwidgets::saveWidget() to fail when
  run in a Shiny session.
• text3d() and mtext3d() did not pass the cex argument to
  plotmath3d() (pull request #384).
• polygon3d() failed when given exactly 3 points (issue #388).
• snapshot3d() failed on Windows with some versions of webshot2 (issue #391).
• Fixed issues caused by misuse of dev.off() using new function
  safe.dev.off().
• Fixed issue with warning() call reported by CRAN.