Reduce memory leaks when switching credocuments

CRE cache, hyphdict and fonts can be initialized only once
when first credocument is opened. Previously, they were
recreated for each document, and as previous instances were probably
not free'd, this caused memory leaks.

frontend/document/documentregistry.lua

@@ -28,6 +28,11 @@ function DocumentRegistry:getProvider(file)
 end
 
 function DocumentRegistry:openDocument(file)
+    -- force a GC, so that any previous document used memory can be reused
+    -- immediately by this new document without having to wait for the
+    -- next regular gc. The second call may help reclaming more memory.
+    collectgarbage()
+    collectgarbage()
     if not self.registry[file] then
         local provider = self:getProvider(file)
         if provider ~= nil then