Gets the diagnostics of the last render: one warning, code RenderingOperationSkipped, for each kind of display-list
operation this backend does not draw yet and left out, and one information entry, code BackendNativeGlyphsUnavailable,
when glyphs of a TrueType or OpenType program were drawn from their outlines because DirectWrite could not read the program.
public IReadOnlyList<Diagnostic> Diagnostics { get; }