There were a few problems flushing texture overrides so that sliced
textures would not work:
* In _cogl_material_set_layer_texture it ignored the 'overriden'
parameter and always set texture_overridden to FALSE.
* cogl_texture_get_gl_texture wasn't being called correctly in
override_layer_texture_cb. It returns a gboolean to indicate the
error status but this boolean was being assigned to gl_target.
* _cogl_material_layer_texture_equal did not take into account the
override.
* _cogl_material_layer_get_texture_info did not return the overridden
texture so it would always use the first texture slice.
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