Fix incorrect index data size

gpu.c

@@ -778,7 +778,7 @@ void gpu_init(gpu_ctx_t *ctx, const gpu_config_t *config, model_t model)
 
 	// Set up staging, vertex and index buffers
 	const size_t vertex_data_size = model.vertex_count * sizeof(vertex_t);
-	const size_t index_data_size = model.index_count * sizeof(vertex_t);
+	const size_t index_data_size = model.index_count * sizeof(index_t);
 	VkBuffer staging_buf;
 	VkDeviceMemory staging_buf_mem;
 	const size_t staging_buf_size = MAX(vertex_data_size, index_data_size);