[nostalgia] Collapse NostalgiaCore down to a single library and cleanup impl data access

src/nostalgia/core/glfw/core.cpp

@@ -68,9 +68,10 @@ bool buttonDown(Key) noexcept {
 	return false;
 }
 
-void shutdown(Context *ctx) noexcept {
+ox::Error shutdown(Context *ctx) noexcept {
 	const auto id = ctx->windowerData<GlfwImplData>();
 	glfwSetWindowShouldClose(id->window, true);
+	return OxError(0);
 }