Previously, when Stop was called on a Cuda timer, it would record a stop
event but it would not synchronize it at that time. Instead, the
synchronize was only called when GetElapsedTime was called. The problem
is that the time of the event is only marked when synchronize is called.
Thus, if the event completed before GetElapsedTime was called, it would
record the time from when the event acutally happened to the time when
GetElapsedTime was called as part of the elapsed time, which is
incorrect.

Fix the problem by synchronizing when Stop is called. Although this
makes the Timer more invasive, generally using the Timer can cause
synchronization to happen. This behavior is consistent with the Timer
implementation for other devices.