Binary shader support has been removed in commit c2f974f2a5 ([core]
Remove binary shader support). This left-over parameter is not used
anywhere anymore.
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).