FFmpeg: Split command to list does not work

[iLLiCiTiT]

Issue

• PR FFmpeg: Subprocess arguments as list #2032 broke some execution of ffmpeg commands when split_command_to_list is used paths (mainly executable path) may not be correctly resolved

Changes

• don't use split_command_to_list for unknown string commands
• removed split_command_to_list
• added shell=True where ffmpeg command is not list of arguments but string

NOTE

• this is step back to previous state but now it should be more secure as we handle paths correctly

Related to #2045