Change ResourceManager::getNextHandle() to be lock-free by using atomic counter

Description

This task is with regards to the discussion made on thread "Development Forums/Developer Talk/Optimizing Away the Use of Strings as Handles" and my comment on taking on this task: http://www.ogre3d.org/forums/viewtopic.php?f=4&t=75131&sid=1db6aa49d653dd45288238e89f10807a&p=481914#p481912

Environment

None

Assignee

M

Reporter

AbiU

Time tracking

3h

Components

Fix versions

Affects versions

Priority

Minor
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