Indigo 0.12.0

What has and has not been run, plainly: the host tests pass (4035 checks), the warnings sweep is clean, and the 3DS build links against Wolfram v0.37.0. The video poster has not been seen on a 3DS or in Azahar; the snapshot harness renders it with a stand-in font, so the widths there are approximate.

Added

• A video in a post shows its poster frame, decoded at the size it is drawn (400 px at most), with a line saying it cannot play on the 3DS. The poster and its shape come from Wolfram v0.37.0's wf_post_embed. (#69)