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/gstcudafilter.c

@@ -26,6 +26,7 @@
 #include "gstnvrtcloader.h"
 #include "gstcudanvrtc.h"
 #include "gstcudaconvert.h"
+#include "gstcudascale.h"
 
 /* *INDENT-OFF* */
 const gchar *nvrtc_test_source =
@@ -50,4 +51,6 @@ gst_cuda_filter_plugin_init (GstPlugin * plugin)
 
   gst_element_register (plugin, "cudaconvert", GST_RANK_NONE,
       GST_TYPE_CUDA_CONVERT);
+  gst_element_register (plugin, "cudascale", GST_RANK_NONE,
+      GST_TYPE_CUDA_SCALE);
 }