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`.
- Add explanatory comment
- Remove dead/unused Android test
- Increase image diff threshold to .05 to accommodate slightly different versions of "PingFang" between different test platforms
Core test uses stubbed "glyph.pbf" without Chinese glyphs
Darwin test relies on locally available "PingFang" font.
Android test relies on locally available "Droid" font. 'expected.png' is NOT correct b/c I haven't figured out how to run unit tests on Android yet.
