Add FFmpeg `-hwaccel` support

[amyipdev]

Producing the final video is painfully slow - only 94 fps on my R7 3700X. This could be aided by using FFmpeg's -hwaccel option to utilize OpenCL.

[Dreiparrent]

Hey @amyipdev, we at JumpCutter do this with our renderer. It's fast if you have an gpu and faster with an Nvidia card. But without a card it's quite slow since I added support for weird video formats and apple'a crappy encoders.

Luckily JumpCutter will be releasing an update soon with a new renderer for those without the hardware as well. Stay tuned for that, and feel free to use our renderer wherever you need.

[amyipdev]

Isn't JC closed source and paid?