perf(objects): Deimos, Phobos and Dimorphos hide their back faces and get depth groups; a drag goes from 23 frames over 33 ms to none on an iPad

[alowpoly]

Deimos, Phobos and Dimorphos drew both sides of every face: their own stylesheets set backface-visibility: visible where every other triangle body hides the far side. The browser drew and sorted all 1,600 faces of a Mars moon on each frame, and the bake could build no depth groups for them ("unresolved static facing"). Their stylesheets now hide back faces, and the bake builds the groups: 16, 6 and 7.

Proves the change: iPad, the same drag of 390 frames, frames over 33 ms and the longest frame: Deimos 23 and 140 ms to 0 and 24 ms; Phobos 24 and 160 ms to 1 and 37 ms; Dimorphos 0 and 30 ms after. With only the stylesheet changed, before the groups, Deimos had 10 and 69 ms. A drag with half of Deimos's faces removed had 0 and 20 ms, and Tethys, with 2,000 faces and groups, 0 and 27 ms. After a Deimos dataset pick the camera's turn ran about 20 frames of 45 to 70 ms; now none passes 44 ms. Phobos's repeat dataset switches go from 166-187 to 143-147 ms. iPad screenshots after a drag show whole bodies.

Clean: the bake's depth-partition and prepared-presentation tests (27 pass).

Limits: the frame of a Deimos dataset switch itself is unchanged, 202 to 229 ms. Dimorphos was not timed before. This branch sits on the Haumea commit of #1351 until that merges. No phone measured.

• Prepared assets published.