[cogl-material] Restore the GL_TEXTURE_ENV_MODE after material_rectangle

The rest of Cogl expects the texture mode to be GL_MODULATE so it
needs to be restored after calling cogl_material_rectangle. Otherwise
cogl_texture_rectangle will fail to blend with the Cogl color properly
and all of the labels will be black.

clutter/cogl/gl/cogl-texture.c

@@ -2595,6 +2595,10 @@ cogl_material_rectangle (CoglFixed        x1,
       GE (glDisable (GL_TEXTURE_2D));
     }
 
+  /* The rest of Cogl expects the texture mode to be GL_MODULATE so we
+     need to restore that */
+  GE( glTexEnvi (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE) );
+
   /* XXX: a bit over precautious. For one we don't support lighting yet
    * so there's no real need to reset the material properties. */
 #if 0