move Gawain library to intern

Before now it lived in source/blender/gpu for convenience. Only a few files in the gpu module use Gawain directly.

Tested on Mac, time to push and test on Windows.

Todo: some CMake magic to make it easy to
#include "gawain/some_header.h"
from any C or H file. Main problem here is the many editors that include GPU_immediate.h which includes Gawain's immediate.h -- is there a way to avoid changing every editor's CMakeLists?

intern/CMakeLists.txt

@@ -33,6 +33,7 @@ add_subdirectory(opencolorio)
 add_subdirectory(mikktspace)
 add_subdirectory(glew-mx)
 add_subdirectory(eigen)
+add_subdirectory(gawain)
 
 if(WITH_GAMEENGINE_DECKLINK)
 	add_subdirectory(decklink)