pango-render: set CLAMP_TO_EDGE wrap mode on glyph_material

The pango renderer was causing lots of override materials to be allocated
because the vertex_buffer API converts AUTOMATIC mode into REPEAT for
backwards compatibility. By explicitly setting the wrap mode to
CLAMP_TO_EDGE when creating the glyph_material then the vertex_buffer
API will leave it untouched.

pango/cogl-pango-render.c

@@ -139,6 +139,8 @@ cogl_pango_renderer_init (CoglPangoRenderer *priv)
   cogl_material_set_layer_combine (priv->glyph_material, 0, /* layer */
                                    "RGBA = MODULATE (PREVIOUS, TEXTURE[A])",
                                    NULL);
+  cogl_material_set_layer_wrap_mode (priv->glyph_material, 0,
+                                     COGL_MATERIAL_WRAP_MODE_CLAMP_TO_EDGE);
 
   priv->solid_material = cogl_material_new ();