osc: Pass user-visible piid to OSCSelectObserver

There are two ways to identify a plugin:

 1. The user-visible number, usually named "piid". This is a 1-based
    index into the list of visible, non-channelstrip plugins only.
 2. The plugin id. This is a 1-based index into the list of all plugins as passed to nth_plugin

The sur->plugins vector is used to translate the former into the latter.

Previously, the latter, already translated, index was passed to
OSCSelectObserver to indicate the currently selected plugin. To prepare
for sending the piid of the selected plugin in a feedback message (in
the next commit), this commit moves the translation of piid to plugin id
into OSCSelectObserver.

To emphasize the new meaning of the plug_id variable, it is renamed to
selected_piid. Also, since the piid is 1-based, change it to a uint32_t
and use 0 to mean "no plugin selected" instead of -1.

This commit is expected to subtly change behaviour: When selecting
a different strip, OSCSelectObserver would (via OSC::_strip_select2 and
refresh_strip) send feedback for the plugin with the same plugin id as
the previously selected plugin. Then OSC::_strip_select2 would do the
piid to plugin id translation for the new strip and call set_plugin,
causing a second round of feedback for the plugin with the same piid as
the previously selected plugin.

If the new strip has hidden plugins (i.e. piid and plugin id do not
match), this could mean that you get feedback for two different plugins.

With this commit applied, the translation happens in both cases and the
feedback returned should be for the same plugin twice.

libs/surfaces/osc/osc.cc

@@ -2888,7 +2888,7 @@ OSC::_sel_plugin (int id, lo_address addr)
 			sur->plugin_id = 0;
 			sur->plug_page = 1;
 			if (sur->sel_obs) {
-				sur->sel_obs->set_plugin_id(-1, 1);
+				sur->sel_obs->set_plugin_id(0, 1);
 			}
 			return 0;
 		} else if (id < 1) {
@@ -2924,7 +2924,7 @@ OSC::_sel_plugin (int id, lo_address addr)
 		sur->plug_page = 1;
 
 		if (sur->sel_obs) {
-			sur->sel_obs->set_plugin_id(sur->plugins[sur->plugin_id - 1], sur->plug_page);
+			sur->sel_obs->set_plugin_id(sur->plugin_id, sur->plug_page);
 		}
 		return 0;
 	}

libs/surfaces/osc/osc_select_observer.cc

@@ -77,9 +77,9 @@ OSCSelectObserver::OSCSelectObserver (OSC& o, ARDOUR::Session& s, ArdourSurface:
 	plug_size = plug_page_size;
 	plug_page = sur->plug_page;
 	if (sur->plugins.size () > 0) {
-		plug_id = sur->plugins[sur->plugin_id - 1];
+		selected_piid = sur->plugin_id;
 	} else {
-		plug_id = -1;
+		selected_piid = 0;
 	}
 	_group_sharing[15] = 1;
 	refresh_strip (sur->select, sur->nsends, gainmode, true);
@@ -441,7 +441,7 @@ OSCSelectObserver::send_end ()
 void
 OSCSelectObserver::set_plugin_id (int id, uint32_t page)
 {
-	plug_id = id;
+	selected_piid = id;
 	plug_page = page;
 	renew_plugin ();
 }
@@ -469,7 +469,7 @@ OSCSelectObserver::renew_plugin () {
 void
 OSCSelectObserver::plugin_init()
 {
-	if (plug_id < 0) {
+	if (!selected_piid) {
 		plugin_end ();
 		return;
 	}
@@ -480,7 +480,7 @@ OSCSelectObserver::plugin_init()
 	}
 
 	// we have a plugin number now get the processor
-	std::shared_ptr<Processor> proc = r->nth_plugin (plug_id);
+	std::shared_ptr<Processor> proc = r->nth_plugin (sur->plugins[selected_piid - 1]);
 	std::shared_ptr<PluginInsert> pi;
 	if (!(pi = std::dynamic_pointer_cast<PluginInsert>(proc))) {
 		plugin_end ();

libs/surfaces/osc/osc_select_observer.h

@@ -89,7 +89,7 @@ class OSCSelectObserver
 	uint32_t nplug_params;
 	uint32_t plug_page_size;
 	uint32_t plug_page;
-	int plug_id;
+	uint32_t selected_piid;
 	uint32_t plug_size;
 	std::vector<int> plug_params;
 	int eq_bands;