There is a rare edge case in ransac(), causing the tests
AV1/DatarateTestLarge.ChangingDropFrameThresh/{4,5,6}
to fail when global motion is enabled. The sequence of events is:

* Since GLOBAL_MOTION_TYPES = 3, we try to generate a ROTZOOM model
* A model is generated at ransac.c:215, and its first 4 components
  are copied to best_params. The last four components of best_params
  are left as {0, 1, 0, 0}.
* We then finish the trial_count loop, and call find_transformation
  one final time to generate a refined model. But this fails, and
  best_params is not overwritten.
* get_wmtype decides that this is an AFFINE model, since
  wmmat[4] != -wmmat[3] and wmmat[5] != wmmat[2].
* We try to encode this global motion model. But the entropy coder
  cannot encode an AFFINE model, and crashes at daalaboolwriter.h:74
  (with OD_ASSERT(symb != -1))

The fix is to copy 8 entries to best_params regardless of the
model type we are generating, in line with the changes in
https://aomedia-review.googlesource.com/#/c/5589/

Change-Id: I6dbdfb997924f8ddf7ea3a1d557463264ea63cbe