only connect dirty-related signals for layers that are generators

[ioxu]

(Ignoring the fact that there is only a single generator texture_scene presently, namely test_lines.tscn) Adding a non-generator texture_scene, eg debug_9_swatches.tscn causes errors related to missing signals. Erroring on

• instaniation of the texture_scene into a layer (can't connect signal)

  <C++ Error>   Condition "!signal_is_valid" is true. Returned: ERR_INVALID_PARAMETER
  <C++ Source>  core/object.cpp:1503 @ connect()
  <Stack Trace> layerManager.gd:194 @ connect_layer_signals()
              layerManager.gd:67 @ add_layer()
              ui.gd:280 @ _on_addLayer_TextureButton_pressed()

• dirtying of layers by the ui (no method to handle signal sent to layer)

  <C++ Source>  core/object.cpp:1260 @ emit_signal()
  <Stack Trace> ui.gd:181 @ _on_resolution_menu_button_item_pressed()

[ioxu]

https://github.com/ioxu/microscratches-generator/blob/a8dd0184ad1c72560dad39cddea2866cd272fe2b/scenes/layerManager.gd#L193-L194

Only connect these signals if the layer is a generator? or has method is_dirty() ?

[ioxu]

func connect_layer_signals(layer)->void:
    layer.connect( "selected", self, "_on_layer_selected" )
    layer.connect( "deselected", self, "_on_layer_deselected" )
    layer.connect( "layer_visiblity_toggled", ui_root, "_on_layer_visibility_toggle")
    if layer_is_generator( layer ):
        layer.texture_scene.connect( "dirty", layer,  "_on_texture_scene_dirty" )
        ui_root.connect( "dirtied_by_ui", layer.texture_scene, "_on_set_dirty" )

Seems to work, no more errors.