Improved reading 16-bit TGA images with colour

[Yay295]

Looks like a typo in the mode name. The only other place "BGR;5" is used is in Unpack.c where it's an alias for "BGR;15".

The documentation I found says that it should actually be "BGRA;16" though, which isn't a mode we support.

[Yay295]

Actually, I missed it; "BGRA;15" is supported.

[radarhere]

While the test suite now passes, if you actually run

with Image.open("Tests/images/p_16.tga") as im:
    im.save("out.png")

you get a transparent image.

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. I've created Yay295#20 to fix this. I've also updated the tests to compare with RGBA images to prevent this problem from slipping by again.