graphics/vulkan-tools: unbreak vkcube-wayland on non-tiling compositors

Affects at least wayfire, labwc, plasma5-kwin.

$ vkcube-wayland
Segmentation fault
(lldb) bt

• thread #1, name = 'vkcube-wayland', stop reason = signal SIGSEGV: invalid address (fault address: 0x128000)
  • frame #0: 0x00000008004811f3 libc.so.7`memcpy at memmove.S:314 frame #1: 0x00000000002097f4 vkcube-wayland`demo_update_data_buffer(demo=0x00007fffffffd258) at cube.c:894:5 frame #2: 0x0000000000213e4f vkcube-wayland`demo_draw(demo=0x00007fffffffd258) at cube.c:1043:5 frame #3: 0x000000000020c3fc vkcube-wayland`demo_run(demo=0x00007fffffffd258) at cube.c:2757:13 frame #4: 0x000000000020aa16 vkcube-wayland`main(argc=1, argv=0x00007fffffffe038) at cube.c:4267:5 frame #5: 0x0000000000208340 vkcube-wayland`_start(ap=<unavailable>, cleanup=<unavailable>) at crt1_c.c:75:7