Fix P -> PA conversion

[RedShy]

Resolves #6336

I fixed the function p2pa() in the file Convert.c by multiplying in[0] with 4 like other similar conversion functions do in the same source file.
I also added a test to cover the conversion "P" -> "PA".

[Lucas-C]

Great, precise fix!

Looks legitimate to me.

Have you checked that the transparency of the resulting image can be seen with some image viewer ? (a web browser or your OS default one)

[RedShy]

Have you checked that the transparency of the resulting image can be seen with some image viewer ? (a web browser or your OS default one)

Yes. I ran this code with and without the fix I added

Image.open("black.png").convert("PA").convert("RGBA").save("converted.png")

The original image is half black, half transparent

Without the fix, the resulting image is all transparent:

With the fix the image is like the original: half transparent and half black

[radarhere]

Some history.

#3728 added p2pa with in[0] * 4.
During the release of Pillow 6.0.0 though, this function caused a problem. Part of the fix was to change it to in[0]. I made that change without being able to iteratively test the fix myself though, so it is believable that that line did not actually need changing to fix Pillow 6.0.0.