Add support for multi-line text to SkiaRenderContext (#1538)

[Jonarw]

Fixes #1538.

Checklist

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

Changes proposed in this pull request:

• Add support for multi-line text to SkiaRenderContext using the StringHelper.SplitLines(...) method
• Add an additional example showing multi-line text in combination with rotation and alignment
• Add an additional SkiaSharp-based SVG export test based in the new example to also cover the code path without text shaping

@oxyplot/admins

[VisualMelon]

This feels wrong, but I can't tell from looking at the examples (e.g. Issue 1545 example). It reads like it is adding extra line spacing for the 'last' line (i.e. lineHeight is the sum of the line height and spacing). Sorry if I've just got myself confused (this is something that OxyPlot.Pdf gets wrong, and I think Svg does as well but will be fixed with #1531)

[Jonarw]

Well paint.GetFontMetrics(out _) returns the suggested line height. To get the total height of the text block we multiply this by the number of lines, so for a text with one line nothing changes, for two lines we get double the height etc. Sounds right to me? Am I missing something?

[VisualMelon]

Now I am confused...

My reading of the GetFontMetrics method documention is that it returns the line height (i.e. |descender| + |ascender|) plus the line gap. This would mean that the height of a box with 1 line of text should be less than half of the height of a box with 2 lines of text. If the line gap is small then this won't be noticeable (and will dimish further with more lines). However, the opposite seems to happen with WinForms (more lines means shorter boxes by number of lines), so clearly I'm missing something.

[VisualMelon]

I guess this is just part and parcel of not really having a specification for the MeasureText method.

[Jonarw]

Yes I think your last point is probably the key issue: It is not really specified what height the MeasureText method should actually return. I interpreted this as the line height in SkiaRenderContext, as that happened to coincide fairly well with what CanvasRenderContext gives, but other interpretations are also valid and possibly make more sense.

[VisualMelon]

Given I can't tell the difference, I'd suggest we pretend I never said anything, and instead we open a new issue discussing the possibility of qualifying MeasureText. We are going to have to write custom implementations of multi-text support for WPF/WinForms anyway to support text-alignment properly, so it would be good to nail down the requirements, and perhaps put the text-alignment code in a common location. That way, the individual render contexts only have to worry about measuring and drawing single lines of text. Nope... they have to measure whole blocks otherwise you can't observe line-gap (if it's an issue), but not render them.

[VisualMelon]

Good to merge in my opinion.