Add depth buffering

gpu.h

@@ -31,6 +31,9 @@ typedef struct {
 	VkExtent2D swapchain_extent;
 	uint32_t swapchain_img_count;
 	VkImageView swapchain_img_views[SWAPCHAIN_IMG_BUFFER_SIZE];
+	VkImage depth_img;
+	VkDeviceMemory depth_img_mem;
+	VkImageView depth_img_view;
 	VkFramebuffer framebuffers[SWAPCHAIN_IMG_BUFFER_SIZE];
 	VkShaderModule vert_shader;
 	VkShaderModule frag_shader;