considering visible page count in readerrolling

This should fix #660.

frontend/apps/reader/modules/readerrolling.lua

@@ -238,7 +238,8 @@ function ReaderRolling:onGotoViewRel(diff)
         end
         self:gotoPos(self.current_pos + pan_diff)
     elseif self.view.view_mode == "page" then
-        self:gotoPage(self.current_page + diff)
+        local page_count = self.ui.document:getVisiblePageCount()
+        self:gotoPage(self.current_page + diff*page_count)
     end
     return true
 end

frontend/document/credocument.lua

@@ -361,6 +361,10 @@ function CreDocument:setFloatingPunctuation(enabled)
     self._document:setIntProperty("crengine.style.floating.punctuation.enabled", enabled)
 end
 
+function CreDocument:getVisiblePageCount()
+    return self._document:getVisiblePageCount()
+end
+
 function CreDocument:setVisiblePageCount(new_count)
     DEBUG("CreDocument: set visible page count", new_count)
     self._document:setVisiblePageCount(new_count)