set zoom mode to contentwidth after rotating screen to landscape

As most pages are portrait there is no reason to fit "portrait" page
on "landscape" screen. And "pagewidth" is used less often than "contentwidth".
So "contentwidth" is choosed to set zoom mode on landscape screen.

frontend/ui/reader/readerview.lua

@@ -138,6 +138,10 @@ function ReaderView:onSetScreenMode(new_mode)
 		Screen:setScreenMode(new_mode)
 		self.ui:handleEvent(Event:new("SetDimensions", Screen:getSize()))
 	end
+
+	if new_mode == "landscape" and self.document.info.has_pages then
+		self.ui:handleEvent(Event:new("SetZoomMode", "contentwidth"))
+	end
 	return true
 end