TGA color map entry size is only 1 byte

[radarhere]

In the TGA format, color map entry size is 1 byte.

Color map specification (field 4)
has three subfields:

• First entry index (2 bytes): index of first color map entry that is included in the file
• Color map length (2 bytes): number of entries of the color map that are included in the file
• Color map entry size (1 byte): number of bits per pixel

It is currently read as 2 bytes though -

Pillow/src/PIL/TgaImagePlugin.py

Line 113 in b01fd46

start, size, mapdepth = i16(s, 3), i16(s, 5), i16(s, 7)

This PR updates it to 1 byte rather than 2 - and as per #4614, it is written as s[7] rather than i8(s, 7).

The reason that this hasn't been noticed is because the next byte is the 'X-origin', and I presume this is usually zero. As a test case, I've copied one of the existing test images and hexedited it so this isn't the case.

Credit to https://stackoverflow.com/a/63156250/4093019 for pointing this out, linked to from #4830