on finish of pan gesture when scrolling, do partial refresh

it was set to a full refresh. However, we want to behave as if in
non-scrolling mode and issue a partial refresh. That might get
updated to a full refresh if the full-refresh counter has reached
the limit - which is configurable.

frontend/apps/reader/modules/readerpaging.lua

@@ -344,8 +344,8 @@ function ReaderPaging:onPanRelease(arg, ges)
         end
     else
         self.last_pan_relative_y = 0
-        -- trigger full refresh
-        UIManager:setDirty(nil, "full")
+        -- trigger partial refresh
+        UIManager:setDirty(nil, "partial")
     end
 end