Display a revert icon on ColorPicker's old sample

This denotes that the color can be clicked to revert to the old color.

doc/classes/ColorPicker.xml

@@ -186,6 +186,9 @@
 		<theme_item name="sample_bg" data_type="icon" type="Texture2D">
 			Background panel for the color preview box (visible when the color is translucent).
 		</theme_item>
+		<theme_item name="sample_revert" data_type="icon" type="Texture2D">
+			The icon for the revert button (visible on the middle of the "old" color when it differs from the currently selected color). This icon is modulated with a dark color if the "old" color is bright enough, so the icon should be bright to ensure visibility in both scenarios.
+		</theme_item>
 		<theme_item name="screen_picker" data_type="icon" type="Texture2D">
 			The icon for the screen color picker button.
 		</theme_item>

editor/editor_themes.cpp

@@ -2170,6 +2170,7 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 	theme->set_icon("shape_rect_wheel", "ColorPicker", theme->get_icon(SNAME("PickerShapeRectangleWheel"), EditorStringName(EditorIcons)));
 	theme->set_icon("add_preset", "ColorPicker", theme->get_icon(SNAME("Add"), EditorStringName(EditorIcons)));
 	theme->set_icon("sample_bg", "ColorPicker", theme->get_icon(SNAME("GuiMiniCheckerboard"), EditorStringName(EditorIcons)));
+	theme->set_icon("sample_revert", "ColorPicker", theme->get_icon(SNAME("Reload"), EditorStringName(EditorIcons)));
 	theme->set_icon("overbright_indicator", "ColorPicker", theme->get_icon(SNAME("OverbrightIndicator"), EditorStringName(EditorIcons)));
 	theme->set_icon("bar_arrow", "ColorPicker", theme->get_icon(SNAME("ColorPickerBarArrow"), EditorStringName(EditorIcons)));
 	theme->set_icon("picker_cursor", "ColorPicker", theme->get_icon(SNAME("PickerCursor"), EditorStringName(EditorIcons)));

scene/gui/color_picker.cpp

@@ -1029,6 +1029,14 @@ void ColorPicker::_sample_draw() {
 
 		sample->draw_rect(rect_old, old_color);
 
+		if (!old_color.is_equal_approx(color)) {
+			// Draw a revert indicator to indicate that the old sample can be clicked to revert to this old color.
+			// Adapt icon color to the background color (taking alpha checkerboard into account) so that it's always visible.
+			sample->draw_texture(theme_cache.sample_revert,
+					rect_old.size * 0.5 - theme_cache.sample_revert->get_size() * 0.5,
+					Math::lerp(0.75f, old_color.get_luminance(), old_color.a) < 0.455 ? Color(1, 1, 1) : (Color(0.01, 0.01, 0.01)));
+		}
+
 		if (old_color.r > 1 || old_color.g > 1 || old_color.b > 1) {
 			// Draw an indicator to denote that the old color is "overbright" and can't be displayed accurately in the preview.
 			sample->draw_texture(theme_cache.overbright_indicator, Point2());
@@ -1727,6 +1735,7 @@ void ColorPicker::_bind_methods() {
 
 	BIND_THEME_ITEM(Theme::DATA_TYPE_ICON, ColorPicker, bar_arrow);
 	BIND_THEME_ITEM(Theme::DATA_TYPE_ICON, ColorPicker, sample_bg);
+	BIND_THEME_ITEM(Theme::DATA_TYPE_ICON, ColorPicker, sample_revert);
 	BIND_THEME_ITEM(Theme::DATA_TYPE_ICON, ColorPicker, overbright_indicator);
 	BIND_THEME_ITEM(Theme::DATA_TYPE_ICON, ColorPicker, picker_cursor);
 	BIND_THEME_ITEM(Theme::DATA_TYPE_ICON, ColorPicker, color_hue);

scene/gui/color_picker.h

@@ -237,6 +237,7 @@ private:
 
 		Ref<Texture2D> bar_arrow;
 		Ref<Texture2D> sample_bg;
+		Ref<Texture2D> sample_revert;
 		Ref<Texture2D> overbright_indicator;
 		Ref<Texture2D> picker_cursor;
 		Ref<Texture2D> color_hue;

scene/theme/default_theme.cpp

@@ -954,6 +954,7 @@ void fill_default_theme(Ref<Theme> &theme, const Ref<Font> &default_font, const
 	theme->set_icon("shape_rect_wheel", "ColorPicker", icons["picker_shape_rectangle_wheel"]);
 	theme->set_icon("add_preset", "ColorPicker", icons["add"]);
 	theme->set_icon("sample_bg", "ColorPicker", icons["mini_checkerboard"]);
+	theme->set_icon("sample_revert", "ColorPicker", icons["reload"]);
 	theme->set_icon("overbright_indicator", "ColorPicker", icons["color_picker_overbright"]);
 	theme->set_icon("bar_arrow", "ColorPicker", icons["color_picker_bar_arrow"]);
 	theme->set_icon("picker_cursor", "ColorPicker", icons["color_picker_cursor"]);