CLUserEvent fail to register callback with NPE

[swarr]

Commit: f6df1fb
File: Core/src/main/velocity/com/nativelibs4java/opencl/CLEvent.java
line 92: queue.getContext().getPlatform().requireMinVersionValue("clSetEventCallback", 1.1);

This line breaks callbacks for CLUserEvent - it's queue is null, thus NPE is thrown. Possible soultion - create new field CLContext field in CLEvent, create new constructor with CLContext instead of CLQueue; use it if queue is null, but context is available like in case of CLUserEvent; don't forget to init context in constructor with queue in parameters; and finally fix this line by using context but not queue.

Another possible solution - revert changes in this file.