ReaderPaging: allow for page diff greater than 1 in onScrollPageRel (#8836)

Cf. <https://www.mobileread.com/forums/showthread.php?p=4200567>.

frontend/apps/reader/modules/readerpaging.lua

@@ -861,7 +861,11 @@ end
 
 function ReaderPaging:onScrollPageRel(page_diff)
     if page_diff == 0 then return true end
-    if page_diff > 0 then
+    if page_diff > 1 or page_diff < -1  then
+        -- More than 1 page, don't bother with how far we've scrolled.
+        self:onGotoRelativePage(Math.round(page_diff))
+        return true
+    elseif page_diff > 0 then
         -- page down, last page should be moved to top
         local last_page_state = table.remove(self.view.page_states)
         local last_visible_area = last_page_state.visible_area