Wait for sprite to load before parsing tiles

[ryanhamley]

Launch Checklist

• briefly describe the changes in this PR
  • This addresses Inconsistent rendering of a coalesced image expression #10122 by waiting for the sprite to finish loading before parsing tiles. This will ensure that any image expressions embedded in a style will have their sprite images available to them and we can be confident that if an image isn't found, it's because it's not in the sprite, rather than the result of a race condition in which the tile loaded before the sprite.
• write tests for all new functionality
  • in progress
• post benchmark scores

This does not seem to have a substantial impact on tile load performance

• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Ensure sprite has loaded before parsing tiles to avoid race condition when evaluating image expressions</changelog>

cc @tristen

[ansis]

In what case is this.scheduler not defined?

[ryanhamley]

I copied this from above in the DedupedRequest class. Both classes get scheduler from actor.scheduler and since DedupedRequest lists the type as ?Scheduler, I assumed that should apply to this class as well and I should guard against the possibility that actor won't exist or won't have a scheduler property. I think this could be an issue in testing?