mode7 conversion is buggy in no-remap mode

Today's master.

``` superfamiconv -i bug.png -p logo.pal -t logo.chr -M snes_mode7 -B8 --no-remap
  No matching tile for position 248,56
  No matching tile for position 0,88
```

bug.png: https://anonfiles.com/x2D4O3r6z5/bug_png
(github attachments don't work for me, please attach it yourself if you intend to look at this after a long time, so it doesn't disappear)