* [core] Bump gl-js version
* [core] Implement image expression
* [core] Use new image expression
* [core] Coerce image expression to / from string
* [core] Serialize evaluated image
* [core] Pass available images to layout
* [core] Pass images to evaluation context
* [core] Set available flag value based on image availability
* [core] Allow image coercion to boolean to indicate image availability
* [core] Coalesce image expression
* [core] Add image expression to next build system
* [core] Align serialization format and evaluated type with gl-js
* [core] Add images to expression evaluation method
* [core] Add support for Image expression to expression test runner
* [core] Unskip image expression tests
* [core] Update unit tests
* [core] Use image expression in annotation manager
* [core] Add string to ImageExpression conversion
* [core] Add image expression to expression dsl
* [core] Convert tokens for implicitly created Image literal
* [core] Fix clang format
* [core] Split generated style code lines that are over 120 characters
* [core] Add unit test for image expression equality
* [core] Add image property expression evaluation unit test
* [core] Unskip image expression render test
* [core] Skip 'in' expression tests
* [core] Ignore fill-pattern/update-feature-state render test
* [core] Rename Image::serialize to Image::toValue
Refactor out HeadlessFrontend and HeadlessBackend gl independent code to gfx.
Define gl::HeadlessBackend as subclass, instantiated by gfx::HeadlessBackend static factory method.
GL dependent tests are still using gl::HeadlessBackend directly (not through gfx).
- Do not carry it over everywhere as parameter, it is a shared
instance anyway and the lifecycle is pretty much the app lifecycle
from the moment we instantiate a map.
- Rename to BackgroundScheduler because it is a Scheduler that will
do tasks in the background, we don't make assumptions if it is a
thread pool or a single thread.
- Most importantly, remove the dependency from `core` on `platform`.
