Add alpha channel to sprites

[seb3s]

Description

This PR adds an optional alpha channel to sprite commands. There is a corresponding PR in scenic_driver_local to handle that change.

Motivation and Context

This is very useful to apply smooth transitions efficiently without multiplying the sprites themselves.

Types of changes

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [X] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Improvement/refactoring (non-breaking change that doesn't add any feature but make things better)

[seb3s]

Hi @boydm , I've opened a thread on slack with a small demo of the results. If you have time to have a look :-) Cheers, Sébastien