Fix conflict between sal.h and MinGW STL

Moved #include<windows.h> after #include <string> to avoid conflicts
between sal.h and MinGW STL usage of __in and __out as function
parameters.
Fix MSVC warnings

renderer_d3d11.cpp(615): warning C4389: '==' : signed/unsigned mismatch
renderer_d3d11.cpp(1100): warning C4189: 'deviceCtx' : local variable is
initialized but not referenced
Merge pull request #21 from gtbass/master

Fix conflict between sal.h and MinGW STL