render: use internal_features to indicate EGL support

Instead of checking whether the wlr_egl dependencies are available
in the GLES2 code, introduce internal_features['egl'] and check
that field.

When updating the EGL dependency list, we no longer need to update
the GLES2 logic.

meson.build

@@ -96,6 +96,7 @@ features = {
 }
 internal_features = {
 	'xcb-errors': false,
+	'egl': false,
 }
 
 wayland_project_options = ['tests=false', 'documentation=false']