add page status to readerrolling for better page view_mode support

* now when rendering in page view mode, we only keep track of page
number instead of pos inside document. Because using pos inside
document for page view mode is really a bad idea and will lead to
many unsovlabe bugs...

frontend/ui/reader/readerfont.lua

@@ -92,6 +92,9 @@ function ReaderFont:onShowFontMenu()
 	return true
 end
 
+--[[
+	UpdatePos event is used to tell ReaderRolling to update pos.
+--]]
 function ReaderFont:onChangeSize(direction)
 	local delta = 1
 	if direction == "decrease" then