This adds a test for the enqueue_write_buffer_rect() method
in the command_queue class. This method copies a rectangular
region of memory from the host to a device buffer.
This changes the enqueue_nd_range_kernel() method to return an
event object. This allows clients to monitor the progress of a
kernel executing on a device.
This fixes a bug in the event_profiling test case in the
command_queue test. On AMD platforms, the event object
returned from clEnqueueMarker() has no profiling information
associated with it and returns an error code when accessed.
Now, profiling information for a simple write to a device
buffer is checked instead.
