Add Letter.TextRenderingMode, Letter.StrokeColor and Letter.FillColor

[mvantzet]

I suggest adding Letter.TextRenderingMode, Letter.StrokeColor and Letter.FillColor and adding them as constructor arguments. We populating it with currentState.FontState.TextRenderingMode, currentState.CurrentStrokingColor and currentState.CurrentNonStrokingColor inside ContentStreamProcessor. Likely other places as well.

You can use the TextRenderingMode to detect hidden text (see also #446) and process the fill and stroke colors using your own logic, instead of flattening it into Letter.Color.

I suggest keeping Letter.Color, which takes either fill or stroke color based on the TextRenderingMode, using the current logic inside ContentStreamProcessor:

color = currentState.FontState.TextRenderingMode != TextRenderingMode.Stroke
                    ? currentState.CurrentNonStrokingColor
                    : currentState.CurrentStrokingColor;

If this is a good idea, I can create a PR.