Evas GL: support surfaceless make current

Evas GL: support surfaceless make current

Summary:
Evas GL now supports surfaceless make current, where
evas_gl_make_current can be called with sfc parameter NULL.
This closely resembles EGL_KHR_surfaceless_context extension,
where applications that only want to render to client API targets
can make current to NULL surface instead of creating a dummy egl surface.
@feature