Performance optimizations: Concurrency, & move download/styling/layout to out-of-thread.

Draft integration test against Stylist & HURL, shelve due to incomplete Text v2 transition.

Now I'm having trouble with GLUT in test program, switch back to SDL2 this time without GL.

Reconstruct inline tree.

Don't parse unimplemented subgrids, add notes as to how to implement.

Repair imports in app for new APIs.

Finish implementing preprocessing/CSS parsing & integrating everything with test script.

Draft public API for CatTrap.