Colour Palette function reads four channels (AGBR)

- Colour palettes are now read as 1 byte unsigned chars rather than 4
byte integers.
Fixes missing path separator

- Fixes missing path separator combination in _cleanExternalFilename
function.
Fixes texture attribute import

- Texture attribute files are now read properly. An incorrect variable
name has been removed.
Implements current transparency

- The current value of transparency is recorded. This provides possible
future transparency implementations and is not currently fully
implemented.
Implements saving of new filename in Opcode 63

- The external reference record (opcode 63) now saves the new processed
filename (making it compatible with the current filesystem).