Update SDL2 win32 libs
vcproj fix for "Debug" config
Add a copy of FindSDL.cmake from CMake 2.8.11.2, renamed to FindSDL2.cmake

Unfortunately CMake does not yet ship a FindSDL2.cmake, so let's bundle our
own (using the existing FindSDL.cmake as a base)
Port FindSDL2.cmake to actually find SDL2 rather than SDL, and include it in CMakeLists.txt

(following instructions at http://www.cmake.org/Wiki/CMake:How_To_Find_Libraries)