gtk3: Fixed LineTo implementation. Closes #40323.

The function `cairo_stroke` causes the current path to be cleared from
the cairo context. Thus we loose the current position.

We replace is with `cairo_stroke_preserve` in order to preserves the
path within the cairo context.

lcl/interfaces/gtk3/gtk3objects.pas

@@ -2315,7 +2315,7 @@ begin
   end else
     cairo_line_to(pcr,X+PixelOffset, Y+PixelOffset);
 
-  cairo_stroke(pcr);
+  cairo_stroke_preserve(pcr);
   Result := True;
 end;