Video core: Add utility class for vector operations.

I wrote most of this for ppsspp, so I hold full copyright over it.
In addition to the original release in ppsspp, this provides functionality to easily extend e.g. two-dimensional vectors to three-dimensional vectors.

src/video_core/CMakeLists.txt

@@ -2,8 +2,9 @@ set(SRCS    video_core.cpp
             utils.cpp
             renderer_opengl/renderer_opengl.cpp)
 
-set(HEADERS video_core.h
+set(HEADERS math.h
             utils.h
+            video_core.h
             renderer_base.h
             renderer_opengl/renderer_opengl.h)