Design task: T72229
This feature makes it possible to do a viewport render (a.k.a. playblast) by only rendering those frames on which the selected objects have a keyframe.
The frames to render are stored in a BLI_bitmap, which has a bit for each frame set to 0 (skip) or 1 (render). An alternative approach would be to construct a set of all keyframes to render, but that would make both constructing the list and looking up frames in the list more complex.
The only thing this feature does is skip OpenGL rendering of a frame. As a result, 'skipped' frames are still included in the output, but just use the render result of the last-rendered frame. This is exactly what's described in T72229.