Join mix thread before destroy OpenGL env

[stayHF]

in function GLESVideoMixer::~GLESVideoMixer.
we shoud join mix thread before destory opengl env.

Thread 25 Crashed:
0 libobjc.A.dylib 0x0000000198a87bd0 objc_msgSend + 16
1 OpenGLES 0x000000018be978f0 +[EAGLContext setCurrentContext:] + 112
2 mybaby 0x00000001010a047c std::__1::__function::__func, void ()()>::operator()() (GLESVideoMixer.mm:509)
3 mybaby 0x000000010109f6c4 ___ZN9videocore8JobQueue7enqueueENSt3__110shared_ptrINS_3JobEEE_block_invoke (JobQueue.hpp:59)
4 libdispatch.dylib 0x00000001990b93ac _dispatch_call_block_and_release + 24
5 libdispatch.dylib 0x00000001990b936c _dispatch_client_callout + 16
6 libdispatch.dylib 0x00000001990c34c0 _dispatch_queue_drain + 1216
7 libdispatch.dylib 0x00000001990bc474 _dispatch_queue_invoke + 132
8 libdispatch.dylib 0x00000001990c5224 _dispatch_root_queue_drain + 664
9 libdispatch.dylib 0x00000001990c675c _dispatch_worker_thread3 + 108
10 libsystem_pthread.dylib 0x00000001992952e4 _pthread_wqthread + 816
11 libsystem_pthread.dylib 0x0000000199294fa8 start_wqthread + 4

Thread 0:
0 libsystem_kernel.dylib 0x00000001991fb570 __semwait_signal + 8
1 libc++.1.dylib 0x0000000198248cf8 std::__1::thread::join() + 28
2 mybaby 0x000000010109e408 videocore::iOS::GLESVideoMixer::~GLESVideoMixer() (GLESVideoMixer.mm:254)
3 libc++.1.dylib 0x000000019823d764 std::__1::__shared_weak_count::__release_shared() + 64
4 mybaby 0x00000001010ae994 -VCSimpleSession dealloc