Use a GList instead of a BSD list for CoglPipelineSnippetList

Previously CoglPipelineSnippetList was using the BSD embedded list
type with a mini struct to combine the list node with a pointer to the
snippet. This is effectively equivalent to just using a GList so we
might as well do that. This will help if we eventually want to get rid
of cogl-queue.h

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit 54a168f3c7829c427d54ab517533bb9f7384d022)

cogl/cogl-pipeline-snippet-private.h

@@ -31,18 +31,11 @@
 #include <glib.h>
 
 #include "cogl-snippet.h"
-#include "cogl-queue.h"
 
-typedef struct _CoglPipelineSnippet CoglPipelineSnippet;
-
-COGL_LIST_HEAD (CoglPipelineSnippetList, CoglPipelineSnippet);
-
-struct _CoglPipelineSnippet
+typedef struct
 {
-  COGL_LIST_ENTRY (CoglPipelineSnippet) list_node;
-
-  CoglSnippet *snippet;
-};
+  GList *entries;
+} CoglPipelineSnippetList;
 
 /* Arguments to pass to _cogl_pipeline_snippet_generate_code() */
 typedef struct