Version 1.3.31

[maboa]

• A video's export defaults to WebM where the browser can't encode MP4 (Chromium builds without H.264/AAC, such as on Linux). It used to default to WAV, which dropped the picture and failed outright for a video with no sound.
• CI is green again (CI is red on every run: export tests can't encode on the runner, and three tests are timing-sensitive #706, merged in CI green: a video defaults to WebM without MP4, and tests stop assuming one machine #707): tests no longer assume an AAC encoder, one machine's fonts, or its speed.

Refs #706