[Impeller] compute minimum input hint for shared backdrop layer.

[jonahwilliams]

Right now we're defaulting to blurring the entire backdrop, but this can be speed up by computing the union of all saveLayer coverages during the pre-pass.

[gaaclarke]

I'll take a stab at this.

[gaaclarke]

This was attempted in flutter/engine#56006 but it was discovered that we'd have to expand the bookkeeping of the first pass and it was decided that we should get more data about performance before investing in that extra bookkeeping cost and complexity.

[jonahwilliams]

yeah I was wrong :) , we do need to compute the clipRect because that is what shrinks the size of backdrop filters