Created attachment 144894 [details] [review]
Proposed fix for mkv with vaapi

Trying to encode using vaapi fails if the container is mkv. Changing the output
format to .mp4 in the command works.

How to reproduce:

`ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128 -i input.mp4 -vf
'hwupload' -vcodec h264_vaapi -bf 0 output.mkv`

Output:

`Could not write header for output file #0 (incorrect codec parameters ?):
Invalid data found when processing input
Error initializing output stream 0:0 -- 
Conversion failed!`

Expected:

Works without error.

Proposed:

Patch attached.