wayland: plug mutex leak of cursor thread
Fixes this leak of testsprite:
Allocation 0: 40 bytes
0x7f4d47b0d43c: SDL_calloc_REAL+0x45
0x7f4d47d149bc: SDL_CreateMutex_REAL+0x17
0x7f4d47cab5a7: Wayland_StartCursorThread+0x9f
0x7f4d47cae028: Wayland_InitMouse+0xa7
0x7f4d47cb36ea: Wayland_VideoInit+0x187
0x7f4d47bbee85: SDL_VideoInit+0x358
0x7f4d47a01bfe: SDL_InitSubSystem_REAL+0xe8
0x7f4d47a3cdf1: SDL_InitSubSystem+0x19
0x407366: SDLTest_CommonInit+0x108
0x402fd0: SDL_AppInit+0x605
0x7f4d47a8771f: SDL_InitMainCallbacks+0x6a
0x7f4d47d3e6cf: SDL_EnterAppMainCallbacks_REAL+0x43
0x7f4d47a396f4: SDL_EnterAppMainCallbacks+0x48
0x401445: SDL_main+0x33
0x7f4d47a878c6: SDL_RunApp_REAL+0x42
0x7f4d47a317be: SDL_RunApp_DEFAULT+0x37
0x7f4d47a3e769: SDL_RunApp+0x32
0x40146e: main+0x27
0x7f4d47811575: __libc_start_call_main+0x75
0x7f4d47811628: __libc_start_main+0x88
0x400ff5: _start+0x25