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)
• 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