shell: Define COGL_ENABLE_EXPERIMENTAL_API and related defines globally

This allows us to use new Cogl APIs without having to define things in
every .c/.h file manually.

src/st/st-texture-cache.c

@@ -21,8 +21,6 @@
 
 #include "config.h"
 
-#define COGL_ENABLE_EXPERIMENTAL_API /* for cogl_framebuffer_allocate */
-
 #include "st-texture-cache.h"
 #include "st-private.h"
 #include <gtk/gtk.h>