Unity ignores the weight of the first layer in a controller, and forces it to one; here, we replicate that behavior in Merge Animator.
Note that we choose to do this here rather than in NDMF - this is because NDMF-level APIs are trying to accurately represent the data in the animator, and should be able to round-trip even "garbage" data there.
Closes: https://github.com/bdunderscore/ndmf/issues/534Closes: #1472
Changes in 1.9.0 broke existing avatars that used animators with
different default values than the MA Parameters fields. This change
makes overriding the animator defaults require an explicit configuration;
this is technically a change which would require a minor version bump,
but as this is addressing a major-version-level compatibility break in 1.9.0,
we're going to push this out at a minor version this time.
