cogl: Add an initialiser for enable_flags in cogl_begin_gl

This fixes a warning about an uninitialised value. It could also
potentially fix some crashes for example if the enable_flags value
happened to include a bit for enabling a vertex array if no vertex
buffer pointer was set.

cogl/cogl.c

@@ -663,7 +663,7 @@ void
 cogl_begin_gl (void)
 {
   CoglMaterialFlushOptions options;
-  gulong enable_flags;
+  gulong enable_flags = 0;
   int i;
 
   _COGL_GET_CONTEXT (ctx, NO_RETVAL);