#81 Minor improvement in Unicode to PETSCII mapping

[Dag Lem]

The Unicode -> PETSCII mapping is very nice!

I just noticed two characters where the mapping could be improved, see suggestion below.

"none" encoding:
🮕 U+1FB95 CHECKER BOARD FILL -> A6
🮖 U+1FB96 INVERSE CHECKER BOARD FILL -> FF

"screen" encoding:
🮕 U+1FB95 CHECKER BOARD FILL -> 66
🮖 U+1FB96 INVERSE CHECKER BOARD FILL -> 5E

These mappings are actually 1:1.

[Soci/Singular]

Hello!

Agree, I think somehow I got 1FB95 wrong :( It's corrected now in r3269 and 1FB90 (inverse medium shade) got added as well.

Thanks!