snippet: Add a hook for the layer texture coordinate transformation

This adds a hook called COGL_SNIPPET_HOOK_TEXTURE_COORD_TRANSFORM.
This can be used to alter the application of the layer user matrix to
a texture coordinate or it can bypass it altogether.

This is the first per-layer hook that affects the vertex shader state
so the patch includes the boilerplate needed to get that to work.

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

cogl/cogl-pipeline-snippet-private.h

@@ -68,6 +68,12 @@ typedef struct
      NULL */
   const char *return_variable;
 
+  /* If this is TRUE then it won't allocate a separate variable for
+     the return value. Instead it is expected that the snippet will
+     modify one of the argument variables directly and that will be
+     returned */
+  gboolean return_variable_is_argument;
+
   /* The argument names or NULL if there are none */
   const char *arguments;