put svg with xmlns in img src tag

[jkrumbiegel]

This fixes #1537

A small test I made with Luxor, to show that inline svgs are correctly scaled down to fit the column width:

```@example
using Luxor
@svg begin
    background("black")
end 300 300
```

```@example
using Luxor
@svg begin
    background("red")
end 800 800
```

```@example
using Luxor
@svg begin
    background("blue")
end 1500 1500
```

before:

after:

[mortenpi]

Could you also include a few additional test cases here: https://github.com/JuliaDocs/Documenter.jl/blob/master/test/examples/src/man/tutorial.md#including-images-with-mime

It would be good to have a small matrix of cases there, e.g. whether the <svg> has a viewBox or not, if it has width/height or not etc. Quickly reading through the article you linked, it sounded like even with img tags there might be edge cases where the SVG doesn't scale quite right?

[jkrumbiegel]

It would be good to have a small matrix of cases there

Would that be purely visual tests, checked manually? Because I don't think I can write a test that detects a browser's ability to show the embedded svg :)

[jkrumbiegel]

I've made the following changes now:

SVG stays utf-8, with minimal changes to keep it from corrupting the surrounding html. I think that can also be nice if something doesn't look quite as expected, one can still inspect the SVG string manually in the browser, which doesn't work with base64. And base64 seems to inflate the size anyway.

I splice in the xmlns tag if it's not there, I couldn't find reasons why I shouldn't.

I've added a couple test cases on the page you linked.

[jkrumbiegel]

I'd like to bump this as Makie's documentation could really use a simpler way to return svg's from example blocks

[mortenpi]

Sorry @jkrumbiegel, this dropped off my radar. Let's get this wrapped up.

[mortenpi]

LGTM. Do you mind using Documente#master for Makie for a while though? I would like to leave this change for 0.27, but I don't want to rush that. This can easily be done if you commit the docs/Manifest.toml.

[jkrumbiegel]

No I don't mind using master, thanks for helping to push this over the finish line.

[devmotion]

I experienced the same problem with Jupyter notebooks, so maybe it would be useful to add the same fix to IJulia?

xref: https://discourse.julialang.org/t/cairomakie-and-fontconfig/55931/17?u=devmotion

[mortenpi]

Thanks for iterating on this @jkrumbiegel!

[cormullion]

Did this PR fix the "text in SVG images gets corrupted" issue? Still seeing this problem...

[jkrumbiegel]

Yes, but it's only on master for now

[cormullion]

OK, thanks (and for the fix!). I'll put my doc updates aside for a while...

[mortenpi]

I'll put my doc updates aside for a while...

If you have some docs that need this now, I would suggest just using #master for now by committing a docs/Manifest.toml file.