Implement movie encoding / codec parameters dialog

[Jose-Moreno]

Issue Summary

After some discussion with scribblemaniac and particularly after the issue where AVI was being exported to a lower quality than MP4 (despite being an uncompressed format), this proposal aims to address a fundamental problem for professionals looking to export their video from Pencil2D and to be used in video editors or compositors, and that is the codec configuration of quality / compression parameters.

Ideally Pencil2D should expose the FFMPEG flags as parameters to allow the user to handle the encoding quality of supported movie formats in order to fine tune the video output should they choose to do so.

On a very basic level this encoding dialog should considering elements like:

• [ ] FPS
• [ ] Final size output / resize
• [ ] Color depth
• [ ] Alpha channel (RGB 24 bit vs RGBA 32bit)
• [ ] Force aspect ratio (4:3, 16:9, custom)
• [ ] (Please comment on which additional features this dialog should have so I can add them)

[scribblemaniac]

For this dialog we should look to HandBrake. It's basically just an ffmpeg GUI at the end of the day so we can implement all the same features that they have.

Some HandBrake features of interest

Video:

• [ ] Constant quality/constant bitrate/constant file size (ie. two pass) switch with corresponding value input.
• [ ] Encoder
• [ ] Encoder preset
• [ ] Encoder tuning
• [ ] Web optimization (faststart)
• [ ] Cropping

Filters:

• [ ] Grayscale
• [ ] Vertical + horizontal flip
• [ ] Rotate. Consideration needs to be made on how this would work with the dimension options. Possibly limit this to 90º increments so that the presumably fast transpose option can be used.

Audio:

• [ ] Codec
• [ ] Stereo/mono
• [ ] Sample rate
• [ ] Bit rate

Non-handbrake features

Gif:

• [ ] Dithering method
• [ ] Palette generation type. Generate based on the full video, focus on moving parts, or generate per frame.
• [ ] Memory-saver mode (GIF only). Uses two passes instead of one, so all of the frames don't have to be in memory at the same time to generate the palette. I've been debating on whether or not this should be the default or not. It will approximately double the render time on computers that have enough memory, but probably performs better for lower-end systems when rendering large animations.
• [ ] GIF palette. There could be an option to pass this in manually. Doing so would result in optimal memory and speed, but of course you have to generate the palette before hand. Before anyone asks, we cannot use the Pencil2D palette because of antialiasing, blending, and importing which produce many colors beyond the Pencil2D palette.

Other:

• [ ] Additional options. A text field could be used to tweak the ffmpeg command directly. Only for really advanced users, because there is an inherit security risk involved with this. Perhaps this should be limited only to encoder options, which follow a specific format.
• [ ] Presets. Not to be confused with encoder presets, these would be presets that define all of the above options. We could base some of these off of the HandBrake presets, but many of them are overkill. I'm pretty sure nobody will be rendering in "Super HQ 1080p30 Surround" 😋
• [ ] Switch backends. We don't have support for any other backends, or modular backends for that matter right now, but that could change with the movie exporter rewrite (#681).

Some tweaks should be made to the default parameters, including to fix the AVI issue (#1100). I am currently working on this, but we should leave that for a separate discussion.