c2.h cleanup

Move most of the static functions into c2.c itself, and
only keep the public functions in c2.h

src/compton.h

@@ -7,8 +7,6 @@
 
 // === Includes ===
 
-#include "common.h"
-
 #include <math.h>
 #include <sys/select.h>
 #include <limits.h>
@@ -27,6 +25,9 @@
 #include <errno.h>
 #endif
 
+#include "common.h"
+#include "c2.h"
+
 // == Functions ==
 
 // inline functions must be made static to compile correctly under clang: