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iris: Fix zeroing of transform feedback offsets in strange cases.
Some of the dEQP.functional.transform_feedback tests end up doing
the following sequence of operations:
1. BeginTransformFeedback
2. PauseTransformFeedback
3. Draw
4. ResumeTransformFeedback
At step 1, we'd pack 3DSTATE_SO_BUFFER commands saying to zero the
SO_WRITE_OFFSET registers. At step 2, we disable streamout, so step 3
doesn't bother emitting those commands. Then, step 4 re-packs new
3DSTATE_SO_BUFFER commands with offset = 0xFFFFFFFF, saying to continue
appending at the existing offset. This loads the value from the BO as
the offsets - but we never actually zeroed it.
So, just maintain a flag saying "we actually emitted the commands",
and stomp offset back to zero until we emit some.