Bump ImageSharp.Drawing to 1.0.0

[VisualMelon]

I was going to refit ImageSharp completely, but then I realised that ImageSharp.Drawing is fine with version 1.0.0 still, so this just bumps it so that it's not a beta version.

Checklist

• [ ] I have included examples or tests
• [x] I have updated the change log
• [x] I am listed in the CONTRIBUTORS file
• [ ] I have cleaned up the commit history (use rebase and squash)

Changes

• Bump SixLabors.ImageSharp.Drawing dependencies to version 1.0.0
• Add support for LineJoin in OxyPlot.ImageSharp/ImageRenderContext
• Enable nullable in OxyPlot.ImageSharp

@oxyplot/admins

[VisualMelon]

TODO:

• import fixes from #2098 where applicable (zero thickness crash)
• see if we can get line-join to work

[VisualMelon]

TODO bits are done and tests all run; some examples are erroring but not sure if that's new.

[VisualMelon]

Need to think about why fontFamily is nullable in IRenderContext.

[VisualMelon]

Only errors that seem to be generated now are related to missing fonts