Clean up numeric.h and split FacePositionCache from it

I also optiized FacePositionCache a bit: I removed a map
lookup and vector copy from both branches of getFacePosition.

src/content_cao.cpp

@@ -44,6 +44,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "camera.h" // CameraModes
 #include "wieldmesh.h"
 #include "log.h"
+#include <algorithm>
 
 class Settings;
 struct ToolCapabilities;