/screenshot: Default to zoom = 2

Before, it would silently fail. As we already have a default for
Ctrl+F9, we can just use that instead.

src/gui/C4MessageInput.cpp

@@ -764,7 +764,7 @@ bool C4MessageInput::ProcessCommand(const char *szCommand)
 	if (SEqual(szCmdName, "screenshot"))
 	{
 		double zoom = atof(pCmdPar);
-		if (zoom<=0) return false;
+		if (zoom<=0) zoom = 2;
 		::GraphicsSystem.SaveScreenshot(true, zoom);
 		return true;
 	}