Web font substitution for SVG export

[swmal]

Web font substitution for SVG export

SVG text was measured server-side with one font but rendered by the browser with another. Office cloud fonts such as Aptos Narrow are never visible to browsers, which fell back to wider fonts, so text overflowed and axis labels were miscalculated, especially on Linux.

Changes

• New FontRenderTarget (Document/Web) and OpenTypeFontEngine.GetFamilyForTarget. Configurable WebFontSubstitutions with defaults for Office cloud fonts.
• RenderContext.ForTarget and GetMeasurementShaper. SvgRenderOptions.FontTarget defaults to Web, and PDF is unaffected.
• The substituted font is used for measurement and output alike. Text runs take the font from the measured fragment.
• Drawing rendering no longer uses ITextMeasurer, only ITextShaper. Axes are measured with the axis font instead of the theme heading font.
• Fixed (TextShaper) casts that crashed when metrics-only shapers were used.

Tests

Substitution policy, RenderContext, SVG output, and measurement consistency. All independent of installed fonts.