Patch https://aomedia-review.googlesource.com/c/21783
changed things so that error-resilient frames use the default
global motion parameters as a reference, rather than taking
the reference from the previous frame.

This was implemented by clearing out cm->prev_frame->global_motion
when we have an error-resilient frame. Unfortunately, this causes
an issue: if we have an error resilient frame which isn't stored
into any reference slots, followed by a non-error-resilient frame,
then both frames refer to the same prev_frame. The second frame
then delta-codes against cm->prev_frame->global_motion, but
this was reset to the default values by the intervening
error-resilient frame!

In order to allow the above case to work as intended, expand the
default warp parameter set to a full WarpedMotionParams struct,
and use that as the reference for error-resilient frames.

This also allows us to remove set_default_warp_params, as we
can now just copy directly from default_warp_params.

Change-Id: I9645615db2700c1d3810e6e42f4f1da626fcd5e3