nvcodec: cuda buffers support

Essentially, this MR rebases the old nvcodec cuda buffers MR (!526 (closed)) from @seungha.yang on top of master, and additionally fixes the issues I found after testing it for a while.

The performance has also been measured and it is about the same as ffmpeg's. It can be tested with the following transcoding pipeline:

gst-launch-1.0 filesrc location=test.mp4 ! qtdemux ! h264parse ! nvh264dec ! cudascale ! "video/x-raw(memory:CUDAMemory),width=1280,height=720" ! nvh264enc ! h264parse ! mp4mux ! filesink location=out.mp4

Which should take about the same time to finish as the following ffmpeg command:

ffmpeg -c:v h264_cuvid -resize 1280x720 -i test.mp4 -c:v h264_nvenc -y out.mp4

sys/nvcodec/gstcudaloader.h

@@ -141,9 +141,9 @@ CUresult CUDAAPI CuDeviceGetAttribute (int *pi,
                                        CUdevice dev);
 
 G_GNUC_INTERNAL
-CUresult CuDeviceCanAccessPeer (int *canAccessPeer,
-                                CUdevice dev,
-                                CUdevice peerDev);
+CUresult CUDAAPI CuDeviceCanAccessPeer (int *canAccessPeer,
+                                        CUdevice dev,
+                                        CUdevice peerDev);
 
 G_GNUC_INTERNAL
 CUresult CUDAAPI CuDriverGetVersion   (int * driverVersion);