Fix #6584: text layout problem in engine preview dialog

With language set to German, there was a problem in this dialog box due
to the long strings in this language. It looks like the call to
`GetStringHeight()` here is using `r.top` to help calculate the
max width, rather than `r.left`.

src/engine_gui.cpp

@@ -108,7 +108,7 @@ struct EnginePreviewWindow : Window {
 
 		EngineID engine = this->window_number;
 		SetDParam(0, GetEngineCategoryName(engine));
-		int y = r.top + GetStringHeight(STR_ENGINE_PREVIEW_MESSAGE, r.right - r.top + 1);
+		int y = r.top + GetStringHeight(STR_ENGINE_PREVIEW_MESSAGE, r.right - r.left + 1);
 		y = DrawStringMultiLine(r.left, r.right, r.top, y, STR_ENGINE_PREVIEW_MESSAGE, TC_FROMSTRING, SA_CENTER) + WD_PAR_VSEP_WIDE;
 
 		SetDParam(0, engine);