Use indexed drawing

main.c

@@ -4,11 +4,14 @@
 #include <math.h>
 
 static const vertex_t vertices[] = {
-	{ { 0.0, -0.5 }, { 1.0, 0.0, 0.0 } },
-	{ { 0.5, 0.5 }, { 0.0, 1.0, 0.0 } },
-	{ { -0.5, 0.5 }, { 0.0, 0.0, 1.0 } },
+	{ { -0.5, -0.5 }, { 1.0, 0.0, 0.0 } },
+	{ { 0.5, -0.5 }, { 0.0, 1.0, 0.0 } },
+	{ { 0.5, 0.5 }, { 0.0, 0.0, 1.0 } },
+	{ { -0.5, 0.5 }, { 1.0, 1.0, 1.0 } },
 };
 
+static const uint16_t indices[] = { 0, 1, 2, 2, 3, 0 };
+
 static void
 key_callback(GLFWwindow *window, int key, int scancode, int action, int mods)
 {
@@ -42,6 +45,8 @@ int main()
 		.window = window,
 		.vertex_data = vertices,
 		.vertex_data_size = sizeof(vertices),
+		.index_data = indices,
+		.index_data_size = sizeof(indices),
 	};
 	gpu_ctx_t gpu;
 	gpu_init(&gpu, &gpu_config);