Image expression

[ryanhamley]

Launch Checklist

This PR implements a new Image expression type and image operator as discussed in #8052 particularly #8052 (comment)

• briefly describe the changes in this PR
  • adds an image expression operator to allow for determining an image's availability before requesting it
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page
• tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes
• tagged @mapbox/gl-native if this PR includes shader changes or needs a native port
  • will need a native port @mapbox/gl-native

Changelog
Add image expression operator which determines if a requested image is available in the map's style or not

[asheemmamoowala]

cc @mapbox/map-design-team @mapbox/studio @kkaefer

[alexshalamov]

@ryanhamley Just a few minor comments.

Question, do you plan to add support for default image that is not in a sprite sheet? Or, would it be handled via onStyleImageMissing callback?

Another question is that, what happens when developer removes image from a sprite sheet while worker is parsing tile? It looks like availableImages and images that are in sprite sheet could be out-of-sync and image operator would not fall-back to default value (e.g., when used in coalesce).

[asheemmamoowala]

nit: no change needed

[ryanhamley]

This was automatically added by my editor. It's pretty standard to end files with a new line isn't it? Most of our other files have a trailing new line.

[asheemmamoowala]

LGTM! 🎉

[ryanhamley]

Question, do you plan to add support for default image that is not in a sprite sheet? Or, would it be handled via onStyleImageMissing callback?

@alexshalamov I'm adding an additional step in a follow-up PR to determine the first requested image in an expression (reading from left to right and assuming that the first image requested is the "most desirable" option) which evaluates to null and returning that image's name so that styleimagemissing will always work as expected.

Another question is that, what happens when developer removes image from a sprite sheet while worker is parsing tile? It looks like availableImages and images that are in sprite sheet could be out-of-sync and image operator would not fall-back to default value (e.g., when used in coalesce).

I think this is analogous to what happens if you asynchronously load an image with addImage. Before this PR, asynchronously loading an image will not cause it to appear in a tile that had requested the image previous to it being loaded. This PR does not change that behavior because it would require reloading tiles every time an image is added or removed, which likely would be distracting and probably a performance hit. I think the best way to deal with both of these situations is still an open question.