New `Backdrop` API

The new Backdrop API provides a more flexible way to configure the drawing. We can do more effects such as progressive blur (in progress).

Please refer to the examples in README.md.

Currently available effects:

Note the refraction amount is a possitive value in the new API, while it was a negative value.

The updated demo has a more natural bottom bar.

Deprecation of the `liquid-glass` module

There is no breaking change for the old APIs.

You can still use the old APIs, but they will be removed before RC.