Add new API functions, tjAlloc() and tjFree(), which allow memory to be allocated...

Add new API functions, tjAlloc() and tjFree(), which allow memory to be allocated and freed using a method of the library's choosing.  At the moment, the primary purpose for this is to avoid allocating/freeing memory across the DLL boundary on Windows.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@634 632fc199-4ca6-4c93-a231-07263d6284db