Fix incorrect stroke color bug in SynPDF

[iadcode]

This fix ensures that the stroke colour is reset when GRestore is called. The incorrect behaviour is triggered by the underline being rendered before the latter stroke.
Before the change

After the change

NB: The underline bug is being submitted as a separate PR (#422)

This behaviour can be tested with code such as

  procedure EmfToPdf(EmfFilename: String; PdfFilename: String);
  const
    C_PDFPPI = 72;
    C_MMperInch = 25.4;
  var
    PDFDocument: TPdfDocument;
    Metafile: TMetafile;
    Scale: Single;
  begin
    Metafile := TMetafile.Create;
    try
      Metafile.LoadFromFile(EmfFilename);
      PDFDocument := TPdfDocument.Create(False, 0, False);
      try
        PDFDocument.DefaultPaperSize := psA4;
        PDFDocument.ScreenLogPixels := C_PDFPPI;
        PDFDocument.AddPage;
        Scale := C_PDFPPI / Trunc((Metafile.Width * 100) / (Metafile.MMWidth / C_MMperInch));
        PDFDocument.Canvas.RenderMetaFile(Metafile, Scale, Scale, 0, 0, tpSetTextJustification, 99, 101, tcAlwaysClip);
        PDFDocument.SaveToFile(PdfFilename);
      finally
        FreeAndNil(PDFDocument);
      end;
    finally
      FreeAndNil(Metafile);
    end;
  end;

And using an EMF such as the one in this zip:
BugsEMF.zip

[synopse]

Update: Why is this not merged yet?

We have merged a lot of fixes from third parties, and it almost every time broke the code working for other users of the library.
PDF generation is a tricky matter, and it is easy to break the existing.

From our point of view:

1. Since we don't actively support mORMot 1.18 any more, you are welcome to switch to mORMot 2 and its https://github.com/synopse/mORMot2/blob/master/src/ui/mormot.ui.pdf.pas unit
2. Fixing for one EMF input is likely to break existing code, which works for most users, so we would rather be very conservative, and only accept pull request with demonstrated compatibility
3. You should also ensure that the code strictly follow mORMot formatting and types, and also that it compiles on both FPC and Delphi.