Add helpful static functions to `PICTDESC`

These static functions below are in winforms. FromIcon might be hard without a method to resolve dependencies added from templates.

public static PICTDESC FromBitmap(Bitmap bitmap, IntPtr paletteHandle = default)
{
    PICTDESC desc = new PICTDESC
    {
        picType = PICTYPE.BITMAP
    };

    desc.Union.bmp.hbitmap = bitmap.GetHbitmap();
    desc.Union.bmp.hpal = paletteHandle;
    return desc;
}

public static PICTDESC FromIcon(Icon icon, bool copy)
{
    PICTDESC desc = new PICTDESC
    {
        picType = PICTYPE.ICON
    };

    desc.Union.icon.hicon = copy ?
        User32.CopyImage(
            icon.Handle,
            User32.IMAGE.ICON,
            icon.Width,
            icon.Height,
            User32.LR.DEFAULTCOLOR)
        : icon.Handle;

    GC.KeepAlive(icon);
    return desc;
}

public static PICTDESC FromMetafile(Metafile metafile)
{
    PICTDESC desc = new PICTDESC
    {
        picType = PICTYPE.ENHMETAFILE
    };

    desc.Union.emf.hemf = metafile.GetHenhmetafile();
    return desc;
}

Winforms Tracking: https://github.com/dotnet/winforms/issues/7468

No source file or test is named in the issue. Start by locating the PICTDESC definition and comparing the requested static functions with the WinForms tracking issue; done means the three helpers are available with the shown bitmap, icon, and metafile behavior and the relevant tests or validation pass.