render/vulkan: always wait for last stage to complete before rendering

When we have multiple command buffers in flight, we need to make
sure we don't start rendering before the previous texture uploads
are complete.

include/render/vulkan.h

@@ -195,6 +195,7 @@ struct wlr_vk_renderer {
 
 	struct {
 		struct wlr_vk_command_buffer *cb;
+		uint64_t last_timeline_point;
 		struct wl_list buffers; // wlr_vk_shared_buffer.link
 	} stage;