Deprecates transparent Option

[bigtimebuddy]

Fixes #7066

This clears up the ambiguity of the recent transparent changes by creating two distinct options for Application and Renderer. It's easier to backward support transparent by not overloading it with new meaning.

• contextAlpha the readonly property for creating the canvas.getContext
• backgroundAlpha the amount of background alpha from 0 to 1

The option transparent is marked as deprecated but still supports backward compatibility by setting contextAlpha and backgroundAlpha.

Broken
https://pixijs.io/examples/#/demos-basic/transparent-background.js

Fixed
https://pixijs.io/examples/?v=dev-context-alpha#/demos-basic/transparent-background.js

[ShukantPal]

I find adding a context prefix to "alpha" distasteful, especially since other context fields aren't. But I'll be a stoic. 👍

[bigtimebuddy]

@SukantPal backgroundAlpha does not work with canvas. Did you test that?

Also, I hear you on the context prefix. On the plus tho, it's unlikely a prop that devs will use much since it has some subtle pref side effects in some environments.