Add the ability to replace colors about lunasvg

[Calinou]

I believe color conversion should be done by editing the SVG input itself, then loading the edited string in memory as a SVG.

[Calinou]

@Calinou Any thoughts?

I'm not sure if this should be part of the core API, but I guess it's not much code in the library so it's probably fine.

[sammycage]

Hello ... please can you try to clarify your question? 🙏🙏🙏

[fire]

Close as a duplicate of #48.

[sammycage]

Color conversion can be easily done this way.

class MyCanvas : public CanvasBase
{
public:
    MyCanvas(int width, int height, Dictionary replace_colors);

... 
    void setColor(const Color& color)
    {
        auto replace_color = color;
        for (int i = 0; i < replace_colors.old_colors.size(); i++) {
            change_lunasvg_paint_color(replace_color, replace_colors.old_colors[i], replace_colors.new_colors[i]);
        }

        CanvasBase::setColor(replace_color);
    }

    void setGradient(const LinearGradientValues& values, const Transform& matrix, SpreadMethod spread, const GradientStops& stops)
    {
        auto replace_color = replace_stop_colors(stops);
        CanvasBase::setGradient(values, matrix, spread, replace_color);
    }

    void setGradient(const RadialGradientValues& values, const Transform& matrix, SpreadMethod spread, const GradientStops& stops)
    {
        auto replace_color = replace_stop_colors(stops);
        CanvasBase::setGradient(values, matrix, spread, replace_color);
    }
... 
};

... 
auto document = Document::loadFromFile("any.svg");
auto canvas = std::make_shared<MyCanvas>(width, height, replace_colors):
auto matrix = Matrix{document->width() / width, 0, 0, document->height() / height, 0, 0};

document->renderToCanvas(canvas, matrix);
... 

@Calinou Any thoughts?