backend-eglnative: Search for the xsettings schema recursively

As recommended by the docs for g_settings_schema_source_get_default:

The returned source may actually consist of multiple schema sources from different directories, depending on which directories were given in XDG_DATA_DIRS and GSETTINGS_SCHEMA_DIR. For this reason, all lookups performed against the default source should probably be done recursively.

Now it's actually found and works again, including subpixel font smoothing.

Closes: gnome-shell#1447 (closed)

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