vo_gpu: hwdec_cuda: Synchronise OpenGL Interop

Previously, there appeared to be implicit synchronisation in the
GL interop path, and we never observed any visual glitches. However,
recently, I started seeing stuttering in the GL path and on closer
examination it looked like read-before-write behaviour where GL
would display an old frame again rather than the current one.

After verifying that disabling hwdec made the problem go away,
I tried adding a cuStreamSynchronize() after the memcpys and that
also resolved the problem, so it's clearly sync related.

cuStreamSynchronize() is a CPU sync and so more heavy-weight than
you want, but it's the only tool we have. There is no mechanism
defined for synchronising GL to CUDA (It looks like there is a way
to synchronise CUDA to EGL but it appears one way and so wouldn't
directly address this problem).

Anyway, empirically, the output now looks the same as with hwdec
off.

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