Show Dear ImGui demo window

src/main.zig

@@ -1,11 +1,14 @@
-const sokol = @import("sokol");
+const ig = @import("cimgui");
 const shader = @import("shader");
+const sokol = @import("sokol");
 
-const slog = sokol.log;
-const sg = sokol.gfx;
 const sapp = sokol.app;
+const sg = sokol.gfx;
 const sglue = sokol.glue;
 const simgui = sokol.imgui;
+const slog = sokol.log;
+
+var show_demo_window = true;
 
 const main_action = blk: {
     var ret: sg.PassAction = .{};
@@ -73,6 +76,12 @@ fn frame() callconv(.c) void {
         .dpi_scale = sapp.dpiScale(),
     });
 
+    // --- UPDATE ---
+
+    if (show_demo_window) {
+        ig.igShowDemoWindow(&show_demo_window);
+    }
+
     // --- MAIN PASS ---
 
     sg.beginPass(.{