mesa: Ensure that Paused is reset to false on EndTransformFeedback.

mesa: Ensure that Paused is reset to false on EndTransformFeedback.

If a client calls BeginTransformFeedback(), then
PauseTransformFeedback(), then EndTransformFeedback(), we need to make
sure that the transform feedback object is not left in a "paused"
state, otherwise the next call to BeginTransformFeedback() will leave
transform feedback paused.

Reviewed-by: Brian Paul <brianp@vmare.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>