Add support for type 15 (extended type) and 16 (4k colour) sprites
Sprite type 16 is now accepted as an input format, by defining the appropriate 4:4:4 colour lookup tables. Type 15 class RGB is handled too with 1/2/4/8/12/15/16/32bpp sprites with and without RGB/BGR reversal. Other changes: Tighten up the check for BTPC files, it turns out BTPC 5 also exists but has an incompatible format, so rather than trying to decode them and fail, fail early. Mention the return code in Documents/CmdBasUse. Change real variable 'rgb' to integer since it's only holding a flag. Tested with a variety of special sprites, and a BTPC 5 file. Version 1.56. Tagged as 'ChangeFSI-1_56'
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