Do not add <base> href to SVG clip paths during toImage

[etpinard]

fixes #3260

@plotly/plotly_js to do this "properly" i.e. w/o doing something hacky in Snapshot.toSVG like

d3.selectAll('[clipPath'].each(function() {
  var s = d3.select(this);
  var cp = s.attr('clipPath');
  s.attr('clipPath'. cp.replace(URL_REGEX, '');
})

I made Drawing.setClipUrl expect gd as third argument. This means we can stash the <base> href on gd._context (instead of on Drawing) and use it in setClipUrl.

[antoinerg]

Looks good to me 💃 but maybe let's wait for @Braintelligence to confirm it fixed his issue.

[alexcjohnson]

I'm happy now, and your update to @Braintelligence's fiddle #3260 (comment) works for me - so 💃 from my side but perhaps we can wait for @Braintelligence to confirm.

[Braintelligence]

Sorry guys, I am on vacation. I can confirm tomorrow, but if my fiddle doesn't behave like described anymore, it should be alright!