[core] Refactor HeadlessFrontend/Backend: GL separation and factory (#14692)

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).

test/api/annotations.test.cpp

@@ -10,7 +10,7 @@
 #include <mbgl/util/run_loop.hpp>
 #include <mbgl/util/color.hpp>
 #include <mbgl/renderer/renderer.hpp>
-#include <mbgl/gl/headless_frontend.hpp>
+#include <mbgl/gfx/headless_frontend.hpp>
 
 using namespace mbgl;