player: force stereo as default when encoding

I think the new default for this option might be rather bad in encoding
mode. For starters, we don't even know what layers an encoder supports
at all.

player/main.c

@@ -422,6 +422,7 @@ int mp_initialize(struct MPContext *mpctx)
         m_config_set_option0(mpctx->mconfig, "load-scripts", "no");
         m_config_set_option0(mpctx->mconfig, "osc", "no");
         m_config_set_option0(mpctx->mconfig, "framedrop", "no");
+        m_config_set_option0(mpctx->mconfig, "audio-channels", "stereo");
         mp_input_enable_section(mpctx->input, "encode", MP_INPUT_EXCLUSIVE);
     }
 #endif