Remove Platform::sceneEglConfig()

With per-screen rendering, every output may have different EGLConfig.
Having a single global EGLConfig doesn't work out well.

This change removes Platform::sceneEglConfig(). It's used primarily to
create the global share context. In hindsight, the global share context
can be created without EGLConfig as it's never made current.

EGL_NO_CONFIG_KHR is part of EGL_KHR_no_config_context extension, which
is supported by both Mesa and NVIDIA driver so it should be safe to make
it mandatory.