RemoteFX Parallel Processing of Tiles

Implementation of WinPR Thread Pool API
RemoteFX: added synchronized buffer pools + usage of thread pool for parallel processing of RemoteFX tiles

Usage of threads in the RemoteFX decoder is turned off by default and can be configured with the following registry keys:

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\Software\FreeRDP\RemoteFX]
"UseThreads"=dword:00000001
"MinThreadCount"=dword:0000000F
"MaxThreadCount"=dword:000000FF

This is an initial attempt at using threads in the RemoteFX decoder, more optimization is to come.