Invert alpha bit for map depth 16

[radarhere]

Suggestion for python-pillow#7965

Looking at 'Color Map Specification' in https://www.dca.fee.unicamp.br/~martino/disciplinas/ea978/tgaffs.pdf, it says

When working with VDA or VDA/D cards it is preferred that you select 16 bits (5 bits per primary with 1 bit to select interrupt control) and set the 16th bit to 0 so that the interrupt bit is disabled

I think setting the 16th bit to 0 actually means it is opaque, not transparent like Pillow is used to.