gdm: don't show fingerprint message right away

Right now we show "(or swipe finger)" at the user login prompt
any time we detect a fingerprint reader.

Checking for the presense of a fingerprint reader isn't really
sufficient for knowing if it is appropriate or not to show the
message, though. Often, a user's fingerprint won't be enrolled
in the system even if the machine has a fingerprint reader.

In this scenario, we end up in a situation where the code will
fade out the message right after fading it in, or worse, fade
out the message while fading it in.

The former case looks flickery and bad, and the latter case
causes the login dialog to lock up since it never completes its
"show prompt" animation and we don't procede with the login
process until after that animation.

If a user is enrolled in the system, the fingerprint pam module
tries to tell the user to swipe their finger.  We never show the
user that message because it's redundant with our own "(or swipe
finger)" message and because it uses techy words like "UPEK" and
"TouchStrip".

This commit changes the code to defer showing "(or swipe
finger)" until the fingerprint pam module forwards us its own
message. This makes it less likely we'll show the message when
fingerprint login won't work, and also removes the fingerprint
animation from the critical path "show prompt" animation.

https://bugzilla.gnome.org/show_bug.cgi?id=660492

js/gdm/loginDialog.js

@@ -922,6 +922,7 @@ LoginDialog.prototype = {
 
                      function() {
                          this._sessionList.close();
+                         this._promptFingerprintMessage.hide();
                          this._userList.actor.show();
                          this._userList.actor.opacity = 255;
                          return this._userList.showItems();
@@ -943,7 +944,18 @@ LoginDialog.prototype = {
     },
 
     _onInfo: function(client, serviceName, info) {
-        // we don't want fingerprint messages with the word UPEK in them
+        // We don't display fingerprint messages, because they
+        // have words like UPEK in them. Instead we use the messages
+        // as a cue to display our own message.
+        if (serviceName == _FINGERPRINT_SERVICE_NAME &&
+            this._haveFingerprintReader &&
+            (!this._promptFingerprintMessage.visible ||
+             this._promptFingerprintMessage.opacity != 255)) {
+
+            _fadeInActor(this._promptFingerprintMessage);
+            return;
+        }
+
         if (serviceName != _PASSWORD_SERVICE_NAME)
             return;
         Main.notifyError(info);
@@ -971,10 +983,10 @@ LoginDialog.prototype = {
                      },
 
                      function() {
-                         if (!this._haveFingerprintReader)
+                         // Show it with 0 opacity so we preallocate space for it
-                             return null;
+                         // in the event we need to fade in the message
-
+                         this._promptFingerprintMessage.opacity = 0;
-                         return _fadeInActor(this._promptFingerprintMessage);
+                         this._promptFingerprintMessage.show();
                      },
 
                      function() {