#741 ffmpeg-vbr encounters permission denied when writing NUL.avi for 2-pass

[Insight]

I've discovered a bug in the command prepared by the latest ffmpeg-VBR plugin for the latest DVD Styler (3.3 beta 4)

The command prepares the first pass output file to go to -pass 1 -y "NUL.avi"

Note that the full path is not specified, so it will default to the location of the executable - in this case C:\Program Files\DVDStyler\bin\NUL.avi

With DVD styler running under a normal user account (without elevated permissions), this directory is by default not accessible, so a permissions error occurs:

[out#0/dvd @ 0000016b348ee240] Error opening output NUL.avi: Permission denied
Error opening output file NUL.avi.
Error opening output files: Permission denied

This error isn't caught, so a few other errors happen downstream of this:

[vost#0:0/mpeg2video @ 00000219f3756a40] Error submitting video frame to the encoder
[vost#0:0/mpeg2video @ 00000219f3756a40] Error encoding a frame: Operation not permitted
[vost#0:0/mpeg2video @ 00000219f3756a40] Task finished with error code: -1 (Operation not permitted)
[vost#0:0/mpeg2video @ 00000219f3756a40] Terminating thread with return code -1 (Operation not permitted)
Conversion failed!

The workaround is to run DVDStyler as an administrator, or to tamper with the permissions of the install directory.

Ideally this first-pass file would be written to the temp directory used for all other files.

I've tried updating ffmpeg-vbr.bat to replace both occurrences of "NUL.avi" with "%out_path%NUL.avi" and it seems to be working, but I don't know enough to proclaim it the correct solution.