Use indexed drawing

gpu.h

@@ -31,9 +31,11 @@ typedef struct {
 	VkFence in_flight;
 	VkSemaphore render_fin;
 	VkSemaphore img_avail;
-	size_t vertex_data_size;
 	VkBuffer vertex_buf;
 	VkDeviceMemory vertex_buf_mem;
+	VkBuffer index_buf;
+	VkDeviceMemory index_buf_mem;
+	size_t index_count;
 } gpu_ctx_t;
 
 typedef struct {
@@ -41,6 +43,8 @@ typedef struct {
 	GLFWwindow *window;
 	const vertex_t *vertex_data;
 	size_t vertex_data_size;
+	const uint16_t *index_data;
+	size_t index_data_size;
 } gpu_config_t;
 
 void gpu_init(gpu_ctx_t *ctx, const gpu_config_t *config);