Scroll chat with ScrollByAmount

The mouse wheel on the chat frames and on the AFK camera's chat moves by
ScrollByAmount instead of looping ScrollUp / ScrollDown. Shift-wheel up
scrolls by the full range, so the oldest line stops on top as it does
with the wheel; ScrollToTop would leave it alone on the bottom line.

modules/afkcam.lua

@@ -74,11 +74,7 @@ pfUI:RegisterModule("afkcam", function ()
     chat:RegisterEvent(ctype)
   end
   chat:SetScript("OnMouseWheel",function()
-    if arg1 > 0 then
-      this:ScrollUp()
-    else
-      this:ScrollDown()
-    end
+    this:ScrollByAmount(arg1)
   end)
   overlay.chat = chat