Accept '+' as substitute for '\' for sprite frames (PK3)

Modifies R_Char2Frame() to handle '+' as '', upon @toaster 's suggestion. This sidesteps the PK3 limitation that lumps can't be named with backslash.

For reference, the ASCII sort order:

2.1 takes any subsequent char from 'A' and numbers that as the frame number (frame = char - 'A'). This change simply returns frame = '\\' - 'A' if '+' is used.

Note that R_Frame2Char() will return backslash no matter what (char = 'A' + '\\') because we discard the '+' substitute moniker. I don't think it's necessary to support both directions.

It's worth noting that ZDoom also does char substitution for PK3. From ZDoom Wiki:

Sprite lumps for the \ frame in a WAD (such as VILE\* for one of the Arch-Vile's healing frames) can be put in a ZIP file, the backslash character just has to be changed to a caret character (^). So, VILE^1 to VILE^8 in a ZIP file will be interpreted as the VILE\1 to VILE\8 lumps. This replacement only works for sprites, any other lump name should not contain backslashes anywhere.