add pixi.js/pixijs to rollup config

[Zyie]

updates the rollup config to make pixi.js and pixijs globals for v8 support

[bigtimebuddy]

Unrelated-ish, but probably okay to change these to PIXI in v8.

    // In PixiJS 7.1.0+ filters namespace was removed, but we'll
    // keep it here for backwards compatibility
    '@pixi/filter-alpha': 'PIXI.filters',
    '@pixi/filter-blur': 'PIXI.filters',
    '@pixi/filter-color-matrix': 'PIXI.filters',
    '@pixi/filter-displacement': 'PIXI.filters',
    '@pixi/filter-fxaa': 'PIXI.filters',
    '@pixi/filter-noise': 'PIXI.filters',