shell-app: Add "discrete_gpu" option when launching apps

And adapt existing callers to the new API. This will allow us to
implement a way to launch applications on the discrete GPU for systems
where an "Optimus" system exists.

https://bugzilla.gnome.org/show_bug.cgi?id=773117

src/shell-app.c

@@ -505,7 +505,7 @@ shell_app_activate_full (ShellApp      *app,
       case SHELL_APP_STATE_STOPPED:
         {
           GError *error = NULL;
-          if (!shell_app_launch (app, timestamp, workspace, &error))
+          if (!shell_app_launch (app, timestamp, workspace, FALSE, &error))
             {
               char *msg;
               msg = g_strdup_printf (_("Failed to launch “%s”"), shell_app_get_name (app));
@@ -549,7 +549,7 @@ shell_app_open_new_window (ShellApp      *app,
    * as say Pidgin.  Ideally, we have the application express to us
    * that it supports an explicit new-window action.
    */
-  shell_app_launch (app, 0, workspace, NULL);
+  shell_app_launch (app, 0, workspace, FALSE, NULL);
 }
 
 /**
@@ -1200,12 +1200,14 @@ app_child_setup (gpointer user_data)
  * shell_app_launch:
  * @timestamp: Event timestamp, or 0 for current event timestamp
  * @workspace: Start on this workspace, or -1 for default
+ * @discrete_gpu: Whether to start on the discrete GPU
  * @error: A #GError
  */
 gboolean
 shell_app_launch (ShellApp     *app,
                   guint         timestamp,
                   int           workspace,
+                  gboolean      discrete_gpu,
                   GError      **error)
 {
   ShellGlobal *global;
@@ -1227,6 +1229,8 @@ shell_app_launch (ShellApp     *app,
 
   global = shell_global_get ();
   context = shell_global_create_app_launch_context (global, timestamp, workspace);
+  if (discrete_gpu)
+    g_app_launch_context_setenv (context, "DRI_PRIME", "1");
 
   ret = g_desktop_app_info_launch_uris_as_manager (app->info, NULL,
                                                    context,

src/shell-app.h

@@ -54,6 +54,7 @@ gboolean shell_app_is_on_workspace (ShellApp *app, MetaWorkspace *workspace);
 gboolean shell_app_launch (ShellApp     *app,
                            guint         timestamp,
                            int           workspace,
+                           gboolean      discrete_gpu,
                            GError      **error);
 
 void shell_app_launch_action (ShellApp        *app,