ShowImage: fix scale-to-fit behavior

* Fix the condition so that big images are fitted to the window on first
time only if the option is set.
* Small images are never stretched to the window size, as this isn't
very useful.

The interaction of this global setting with the zoom buttons is a bit
confusing. Should they be merged?

src/apps/showimage/ShowImageView.cpp

@@ -1630,7 +1630,7 @@ ShowImageView::FitToBounds()
 		return;
 
 	float fitToBoundsZoom = _FitToBoundsZoom();
-	if (!fStretchToBounds && fitToBoundsZoom > 1.0f)
+	if (!fStretchToBounds || fitToBoundsZoom > 1.0f)
 		SetZoom(1.0f);
 	else
 		SetZoom(fitToBoundsZoom);