Fixes https://github.com/flutter/flutter/issues/124703

Rather than applying an opacity layer per picker item, apply them to all partially opaque picker items at once. This dramatically improves performance for impeller, by reducing the number of required subpasses and texture allocations signficantly.

Before:
Doesn't finish, 100s of passes

After:

![image](https://user-images.githubusercontent.com/8975114/231569280-91f55c9a-53a5-4b75-8728-59a4dceebe81.png)

![image](https://user-images.githubusercontent.com/8975114/231569309-7c82e5ff-46c7-4f00-80f0-9b4096aa4b14.png)

See also:

https://github.com/flutter/flutter/issues/124658