[cogl] Remove cogl_{create,destroy}_context from the public API

cogl_create_context is dealt with internally when _cogl_get_default context
is called, and cogl_destroy_context is currently never called.

It might be nicer later to get an object back when creating a context so
Cogl can support multiple contexts, so these functions are being removed
from the API until we get a chance to address context management properly.

For now cogl_destroy_context is still exported as _cogl_destroy_context so
Clutter could at least install a library deinit handler to call it.

gles/cogl-context.c

@@ -39,7 +39,7 @@
 static CoglContext *_context = NULL;
 static gboolean gl_is_indirect = FALSE;
 
-gboolean
+static gboolean
 cogl_create_context ()
 {
   GLubyte default_texture_data[] = { 0xff, 0xff, 0xff, 0x0 };
@@ -133,7 +133,7 @@ cogl_create_context ()
 }
 
 void
-cogl_destroy_context ()
+_cogl_destroy_context ()
 {
   if (_context == NULL)
     return;