Specify that Transparent Windows must have Premultiplied Output Buffers on at least Windows

[ProjectHSI]

Like the title says, this bugged me out for a while and has confusing problems if you assume that your compositor is going to use straight alpha images, this results in weird ghosting artifacts and a wild goose chase.

See the "RGB values conflated with Alpha values on transparent window renderer" thread on the Discord.

[DaneelTrevize]

The TL;DR: of that thread is:
If you change the A value to 0 on line30 of
https://github.com/libsdl-org/SDL/blob/main/test/testsoftwaretransparent.c
You will find the corner squares mix varying intensity of red/pink into whatever's behind them, rather than the expected 'red * 0 = 0'.
If you then change line85 to use SDL_BLENDMODE_BLEND, it works 'as expected', but not intuitively (to some)/consistent with the comment directly preceding it.

[ProjectHSI]

rather than the expected 'red * 0 = 0'.
*from a straight alpha image