Improve support for TGA images in P mode with an RGBA palette

[sungbin1015]

Changes proposed in this pull request:

• Set the palette mode to RGBA for TGA images with 32-bit colour map entries, as is already done for 16-bit entries
• Add a test

TgaImagePlugin has a branch for 32-bit colour maps, but it reads them with the "BGRA" rawmode while leaving the palette mode as "RGB". There's no RGB unpacker for BGRA, so the image opens and then fails as soon as it's loaded:

with Image.open("p32.tga") as im:  # colour-mapped, 32-bit BGRA colour map
    im.load()

  File "src/PIL/Image.py", line 993, in load
    self.im.putpalette(self.palette.mode, mode, arr)
ValueError: unrecognized raw mode

The 16-bit branch right above it already sets self.palette.mode = "RGBA", so I've done the same. The image from the new test then converts to RGBA as (255, 0, 0, 255), (0, 0, 255, 0), which is also what ffmpeg decodes from the same bytes. Noticed while reading the plugin; tests pass locally.

Didn't find this reported. #5400 and #7965 dealt with 16-bit colour maps, and #8158 renames the 16-bit rawmode a few lines up but doesn't touch the 32-bit branch.

[radarhere]

I've pushed a commit to support saving with a 32-bit colour map as well.