Don't pass the whole Pixi object to the Pixi extension

[getkey]

stats.enableExtension('pixi', [PIXI, app]);

Doing this will prevent parts of Pixi from being tree-shaked when using a module bundler. It's a shame since not much from Pixi is needed, only this small enum https://github.com/pixijs/pixi.js/blob/e4d574395aeac5ba0a89b52a47528126bcba529f/packages/constants/src/index.ts#L173-L182

Some solutions:

1. Adding @pixi/constants as a dependency. When I tried it, the whole dependency was imported (not only FORMATS). Pixi will need to add pure annotations to avoid that. In the meantime it would needlessly increase the size of this lib (perhaps terser could help though?).
2. Adding @pixi/constants as an optional peer dependencies. It works well, however it breaks if the user is not using a bundler, as in the example.
3. Inlining the enum from Pixi. It's barbaric but it works!

[ErikSom]

I would say lets go with #3