[css-paint-api] Add "alpha" as an option.

Syntax...

``` js
registerPaint('foo', class {
  static get alpha() { return true; }

  paint() { }
});
```

This is similar to CRC2D alpha context option. If true the backdrop of the rendering context is transparent, if false (default) the backdrop is opaque.

An opaque backdrop may prevent loading occluded images, or simplify drawing operations.
