Add color channel filter to editor texture previews

editor/plugins/texture_editor_plugin.h

@@ -39,6 +39,8 @@
 class AspectRatioContainer;
 class ColorRect;
 class TextureRect;
+class ShaderMaterial;
+class ColorChannelSelector;
 
 class TexturePreview : public MarginContainer {
 	GDCLASS(TexturePreview, MarginContainer);
@@ -47,10 +49,14 @@ private:
 	TextureRect *texture_display = nullptr;
 
 	MarginContainer *margin_container = nullptr;
+	Control *outline_overlay = nullptr;
 	AspectRatioContainer *centering_container = nullptr;
 	ColorRect *bg_rect = nullptr;
 	TextureRect *checkerboard = nullptr;
 	Label *metadata_label = nullptr;
+	Ref<ShaderMaterial> material;
+
+	ColorChannelSelector *channel_selector = nullptr;
 
 	Color cached_outline_color;
 
@@ -61,6 +67,8 @@ protected:
 	void _notification(int p_what);
 	void _update_texture_display_ratio();
 
+	void on_selected_channels_changed();
+
 public:
 	TextureRect *get_texture_display();
 	TexturePreview(Ref<Texture2D> p_texture, bool p_show_metadata);