At present, we do the fallback discrete animation for non-invertible matrices in
ComputedMatrix.animate(). However, according to the spec, we should fallback to
discrete animation for cases like:
1. animation between transform with single non-invertible matrix
2. animation between transform with matched transform functions that have at least
one non-invertible matrix
2. animation between transform with mismatched transform functions that have at
least one non-invertible matrix.
The current implementation only handles the first case.
Moreover, we already have fallback discrete animation procedures in CSS Animation
and Web Animation, so we should be able to not doing any fallback inside the
Animate trait.
In this patch, we let the animation between non-invertible matrices to return Err().
So, we can propagate the Err() to the callers, and let the fallback discrete
animation procedure stay at the Servo_MatrixTransform_Operate, which is ouside
the Animate trait.
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