Get the unlit / unshaded extension when importing GLTF and set it when exporting

(cherry picked from commit e14fa5532bd87addc8c019e2b3770758b36d0c09)

modules/gltf/gltf_document.cpp

@@ -3748,6 +3748,14 @@ Error GLTFDocument::_serialize_materials(Ref<GLTFState> p_state) {
 			d["alphaMode"] = "BLEND";
 		}
 
+		Dictionary extensions;
+		if (base_material->get_shading_mode() == BaseMaterial3D::SHADING_MODE_UNSHADED) {
+			Dictionary mat_unlit;
+			extensions["KHR_materials_unlit"] = mat_unlit;
+			p_state->add_used_extension("KHR_materials_unlit");
+		}
+		d["extensions"] = extensions;
+
 		materials.push_back(d);
 	}
 	if (!materials.size()) {
@@ -3780,6 +3788,11 @@ Error GLTFDocument::_parse_materials(Ref<GLTFState> p_state) {
 		if (d.has("extensions")) {
 			pbr_spec_gloss_extensions = d["extensions"];
 		}
+
+		if (pbr_spec_gloss_extensions.has("KHR_materials_unlit")) {
+			material->set_shading_mode(BaseMaterial3D::SHADING_MODE_UNSHADED);
+		}
+
 		if (pbr_spec_gloss_extensions.has("KHR_materials_pbrSpecularGlossiness")) {
 			WARN_PRINT("Material uses a specular and glossiness workflow. Textures will be converted to roughness and metallic workflow, which may not be 100% accurate.");
 			Dictionary sgm = pbr_spec_gloss_extensions["KHR_materials_pbrSpecularGlossiness"];
@@ -7440,6 +7453,7 @@ Error GLTFDocument::_parse_gltf_extensions(Ref<GLTFState> p_state) {
 	supported_extensions.insert("KHR_lights_punctual");
 	supported_extensions.insert("KHR_materials_pbrSpecularGlossiness");
 	supported_extensions.insert("KHR_texture_transform");
+	supported_extensions.insert("KHR_materials_unlit");
 	for (Ref<GLTFDocumentExtension> ext : document_extensions) {
 		ERR_CONTINUE(ext.is_null());
 		Vector<String> ext_supported_extensions = ext->get_supported_extensions();