[cogl] Add cogl_is_material to cogl-material.h

The COGL_DEFINE_HANDLE macro generates a cogl_is_<type> function
as well, to check whether a CoglHandle opaque pointer is of type
<type>.

The handle for CoglMaterial does not export cogl_is_material() in
its installed header.

clutter/cogl/cogl-material.h

@@ -52,7 +52,16 @@ CoglHandle cogl_material_ref (CoglHandle handle);
  */
 void cogl_material_unref (CoglHandle handle);
 
-
+/**
+ * cogl_is_material:
+ * @handle: A CoglHandle
+ *
+ * Gets whether the given handle references an existing material object.
+ *
+ * Returns: %TRUE if the handle references a #CoglMaterial,
+ *   %FALSE otherwise
+ */
+gboolean cogl_is_material (CoglHandle handle);
 
 /**
  * cogl_material_set_color:

doc/reference/cogl/cogl-sections.txt

@@ -330,6 +330,7 @@ cogl_matrix_scale
 cogl_material_new
 cogl_material_ref
 cogl_material_unref
+cogl_is_material
 cogl_material_set_color
 cogl_material_set_color4ub
 cogl_material_set_color4f