Retrieves the currently set magnification PipelineFilter set on the specified layer.
The magnification filter determines how the layer should be sampled when up-scaled.
The default filter is cogl_pipeline_filter_linear but this can be changed using set_layer_filters.
| this | A Pipeline object | 
| layer_index | the layer number to change. | 
| The magnification PipelineFilter for the specified layer. |