source engine gradient using ISurface

void Renderer::DrawRectGradV(int x, int y, int w, int h, Color from, Color to) {
    float r = (to.R() - from.R()) / h;
    float g = (to.G() - from.G()) / h;
    float b = (to.B() - from.B()) / h;
    float a = (to.A() - from.A()) / h;

    for (int i = 0; i < h; i++) {
        int R = from.R() + r * i;
        int G = from.G() + g * i;
        int B = from.B() + b * i;
        int A = from.A() + a * i;

        DrawRect(x, y + i, w, 1, Color(R, G, B, A));
    }
}

void Renderer::DrawRectGradH(int x, int y, int w, int h, Color from, Color to) {
    float r = (to.R() - from.R()) / w;
    float g = (to.G() - from.G()) / w;
    float b = (to.B() - from.B()) / w;
    float a = (to.A() - from.A()) / w;

    for (int i = 0; i < h; i++) {
        int R = from.R() + r * i;
        int G = from.G() + g * i;
        int B = from.B() + b * i;
        int A = from.A() + a * i;

        DrawRect(x + i, y, 1, h, Color(R, G, B, A));
    }
}