Implement window transparency.

[Apprentice-Alchemist]

The FNF modders want transparent windows. The implementation has not been tested.

[dimensionscape]

Just a side note, SDL's opacity affects the entire window and its content. I thought I would throw that out there since it might not be clear, and because some developers would possibly be interested in setting the actual window transparency(and its background) while leaving the window content opaque (as is already possible for the AIR target).

Unfortunately SDL does not support this at the moment. 😢

[JustJasonLol]

Hilarious

[JustJasonLol]

also how it works?

[EyeDaleHim]

PR works well, saw it mentioned by some other dude

I do not own the image.

[player-03]

Sorry for making a dozen different commits. The online UI only allows editing one file at a time, and I didn't feel like going to the effort of checking it out. (This is why there are so many red Xs above; I had to cancel a bunch of redundant tests.)

Most of the changes were sorting and formatting, but two are worth noting:

• In SDLWindow.cpp, I removed the if statement`. Looking at the documentation, it seems unlikely to ever actually return an error, given that Lime checks the handle before calling it. Plus that's how the rest of the file is written.
• I replaced Window.getOpacity() and Window.setOpacity() with getters and setters, since that's how the rest of the Window class works.

And for future reference, FlashWindow.hx and NativeWindow.hx still aren't fully sorted. Something to do later, and not as part of this pull request.

[player-03]

(was saying "fnf modders need this" necessary..?)

It's always good to know why a change is being suggested. Granted, this wasn't a very thorough explanation, but it's a simple change and apparently the modders in question are happy with the outcome, so why not?

[JustJasonLol]

Omg it exist