hwdec_cuda: Add trivial cuda-copy wrapper

The cuvid decoder already knows how to copy back to system memory
if NV12 frames are requested, and this will happen if the decoder
is used without the hwdec.

For convenience, let's add a wrapper hwdec so people don't have
to explicitly pick the cuvid decoder if they want this behaviour.

options/options.c

@@ -99,6 +99,7 @@ const struct m_opt_choice_alternatives mp_hwdec_names[] = {
     {"rpi",         HWDEC_RPI},
     {"mediacodec",  HWDEC_MEDIACODEC},
     {"cuda",        HWDEC_CUDA},
+    {"cuda-copy",   HWDEC_CUDA_COPY},
     {0}
 };