Allow passing pre-parameters using "-i" to specify where the input file name...

Allow passing pre-parameters using "-i" to specify where the input file name should go in ffmpeg parameters
Should work in proxy and transcoding, for example:
-vaapi_device /dev/dri/renderD128 -hwaccel vaapi -hwaccel_output_format vaapi -i -an -c:v dnxhd
Will insert source file name after "-i".
CCBUG: 378832