Add support for 'ani.res' for arbitrary graphic devices
in 'saveGIF()', 'saveHTML()', and 'saveVideo()'.
Should work for any graphic device function with 'res'
as an argument. In particular should continue to support R's built-in
raster image format devices passed as strings as well as now supporting 'ragg::agg_png()'
and R's built-in devices passed in as a function instead of string.
Still no 'ani.res' support in 'saveLatex()' ('pdf' devices recommended)
nor 'saveSWF()' (flash no longer supported in many browsers)
closes #134