Wayland's Weston reference compositor now has built-in support for video capturing.
The built-in Wayland video capturing utility works with all of the Wayland back-ends (like KMS, X11, and Wayland) and tracks changes/damage to the screen and records that data in an efficient and lossless manner. The files are then dumped to "WCAP" and there is a tool available that allows converting these Wayland/Weston video files to WebM video format or extracting single frames.
Kristian Høgsberg mentioned this Weston Video Capture Tool on the Wayland mailing list
This "WCAP" work can be found in the Weston Git repository