CHR_TRN can still be effective during border fade-out

`CHR_TRN` is normally buffered to SNES WRAM, and only gets transferred during a `PCT_TRN`. However, the transfer point is when the border *finishes* fading (so the transfer is seamless), so it's actually possible to send `PCT_TRN`, then `CHR_TRN` during the fade, and have it work.

*Alfred Chicken* apparently relies on this.

- [x] Investigate timing window
- [ ] Document all this

Note that this also means the border tiles do NOT update immediately upon `CHR_TRN`!