Unify multi-line text alignment on all the platforms

[Jonarw]

Benchmark should be the 'Rendering Capabilities > Multi-line Alignment' and 'Rendering Capabilities > Multi-line Alignment/Rotation' examples.

• WPF - fix horizontal alignment
  

• Windows Forms (and Core.Drawing) - fix horizontal alignment
  

• SkiaSharp
  

• ImageSharp - fixed in Improve ImageSharp.ImageRenderContext text alignment #1553
  

• SVG
  

• OxyPlot.PdfExporter - multiline text is not supported. Given we want to mark this as obsolete anyway, we don't have to worry about this too much I think
  

• OxyPlot.Pdf.PdfExporter - multiline text is not supported. Given we want to mark this as obsolete anyway, we don't have to worry about this too much I think
  

[VisualMelon]

If we reach a happy conclusion with #1551, then the code for performing this layout task could be provided by RenderContextBase. Adjustments will need to be made by the different platforms to account for the different rendering APIs (e.g. OxyPlot.Svg can only render relative to baseline), but this could reduce the individual complexity of each renderer, and generally make the code tidier.

If I can sit down for more than a few minutes I'll try to put a version of this together for OxyPlot.Svg implementation, which really needs #1551 to make sense at all.