Add text string in TextRun

[benoitkugler]

The first big problem I have is with text rendering. In your backend, the render is done glyph by glyph, but both libs takes string to draw text. To try to resolve this string problem, I have added a complementary interface in backend. If rasterizer implements it, it is called instead of actual implementation. So text is drawn as string instead of glyphes. It is less powerfull, but the text is drawn.

I would rather deduce the text string from the current TextRun output. It is almost doable using https://pkg.go.dev/github.com/benoitkugler/[email protected]/backend#FontChars and its Cmap field, but not very easy.
I'll try to simplify this task.

Originally posted by @benoitkugler in #6