ReaderRolling: proper refresh when toggling scroll/page mode

When toggling between scroll and page modes multiple
times, _gotoXPointer() could find the xpointer to be
already positionned and avoid calling setDirty() (so
not enqueuing a whole screen refresh). But the page
will be slightly moved to account for the top margin
disappearing. We need that whole screen refresh in
such cases, so ensure one is enqueued.

frontend/apps/reader/modules/readerrolling.lua

@@ -688,6 +688,8 @@ function ReaderRolling:onChangeViewMode()
     self.ui:handleEvent(Event:new("UpdateToc"))
     if self.xpointer then
         self:_gotoXPointer(self.xpointer)
+        -- Ensure a whole screen refresh is always enqueued
+        UIManager:setDirty(self.view.dialog, "partial")
     else
         table.insert(self.ui.postInitCallback, function()
             self:_gotoXPointer(self.xpointer)