feature request: when downloading plot as png using the modebar button, use the current size of the plot as the size of the png.

[finger563]

Currently it does not use the size of the plot as it has been rendered into the browser. The data ranges and such are all the same, but since the size is different the legend may overlap parts that it did not when viewed in the browser and the aspect ratio may be different.

[cpsievert]

Duplicate of plotly/plotly.js#1576

You should be able to replicate the workaround with a combination of config() and htmlwidgets::JS()

[cpsievert]

Closing since I won't be making any changes in the R package to address this