add standalone glyph cache

I found it will be especially helpful to cache glyph separately
so that glyph caches won't be washed out by a single page cache.

frontend/cache.lua

@@ -1,3 +1,4 @@
+require "dbg"
 --[[
 Inheritable abstraction for cache items
 ]]--
@@ -29,8 +30,14 @@ Cache = {
 	cache_order = {}
 }
 
+function Cache:new(o)
+	o = o or {}
+	setmetatable(o, self)
+	self.__index = self
+	return o
+end
+
 function Cache:insert(key, object)
-	--@TODO add cache for different types of item  09.01 2013 (houqp)
 	-- guarantee that we have enough memory in cache
 	if(object.size > self.max_memsize) then
 		-- we're not allowed to claim this much at all