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`.
There were two PRs that are in mergeable state, yet, had dependencies
between each other, therefore, introduced build break on master for
benchmark target.
* [core] Add format override expression and formatted section to evaluation context
* [core] Add textColor to TaggedString's formatted section
* [core] Add FormatSectionOverrides and introduce overridable properties
* [core] Populate symbol layer paint properties for text sections
* [core] Add benchmark for style that uses text-color override
* [core] Add unit test for FormatOverrideExpression
* [core] Add unit test for FormatSectionOverrides
