Video exporting by calling FFMPEG externally

[OverloadedOrama]

This PR implements the ability to export projects to video formats (mp4, avi, ogv, mkv & webm) using FFMPEG. FFMPEG itself is NOT bundled in Pixelorama, and thus needs to be downloaded as a stand-alone software. Then, you need to insert the file path of FFMPEG's binary in Pixelorama's preferences. I have only tested it on Linux so far. EDIT: Should also work on Windows. In the future we could look into bundling it with GDExtension, if the licenses are compatible. Otherwise, I suppose it could be in the form of an extension.

It works by saving all of the frames as png files in a temp folder, then FFMPEG is called to convert the png files into a video, taking into account the project's fps and the delay of each individual frame. Then, the temporary png files are being deleted.

Implements #658.

Ideas for the future (outside the scope of this PR):

• Perhaps it might be a good idea to use FFMPEG for gif and apng exporting as well, as it is a lot faster than using GDScript.
• The ability to use FFMPEG to create videos from the recorder panel.
• Allow users to change the video codecs used for each container.
• Use FFMPEG to import videos and gif files as individual frames.

This feature could work well if we implement audio support in the timeline (#897), allowing people to use Pixelorama as a fully fledged tool to create animations.

3d.cube.spin.8x.webm

A .webm video exporting directly from Pixelorama.