audio subsystem: openal emitter, minor audio fixes

ResourceManager.h

@@ -9,6 +9,8 @@ http://mozilla.org/MPL/2.0/.
 
 #pragma once
 
+int const null_handle = 0;
+
 enum class resource_state {
     none,
     loading,
@@ -64,6 +66,11 @@ class ResourceManager
 };
 */
 
+using resource_timestamp = std::chrono::steady_clock::time_point;
+
+// takes containers providing access to specific element through operator[]
+// with elements of std::pair<resource *, resource_timestamp>
+// the element should provide method release() freeing resources owned by the element
 template <class Container_>
 class garbage_collector {