Fix switching plugin positions in plugin mode

It expects fast response from backend, which is not what happens
with plugins (using pipes).
For now, just force the change manually
Fixes #671

source/carla_backend.py

@@ -2886,7 +2886,10 @@ class CarlaHostPlugin(CarlaHostMeta):
         return self.sendMsgAndSetError(["replace_plugin", pluginId])
 
     def switch_plugins(self, pluginIdA, pluginIdB):
-        return self.sendMsgAndSetError(["switch_plugins", pluginIdA, pluginIdB])
+        ret = self.sendMsgAndSetError(["switch_plugins", pluginIdA, pluginIdB])
+        if ret:
+            self._switchPlugins(pluginIdA, pluginIdB)
+        return ret
 
     def load_plugin_state(self, pluginId, filename):
         return self.sendMsgAndSetError(["load_plugin_state", pluginId, filename])
@@ -3244,4 +3247,9 @@ class CarlaHostPlugin(CarlaHostMeta):
     def _set_peaks(self, pluginId, in1, in2, out1, out2):
         self.fPluginsInfo[pluginId].peaks = [in1, in2, out1, out2]
 
+    def _switchPlugins(self, pluginIdA, pluginIdB):
+        tmp = self.fPluginsInfo[pluginIdA]
+        self.fPluginsInfo[pluginIdA] = self.fPluginsInfo[pluginIdB]
+        self.fPluginsInfo[pluginIdB] = tmp
+
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