Our `morphNormal` GLSL function treats each normal target as a
displacement from the base normal (similar to the glTF spec) but the
normal that is extracted from the tangent frame is actually an absolute
normal.
In other words, if b is the base normal, we were doing:
b = b + w0 * n0 + w1 * n1 + ...
This is obviously wrong, since the base normal has an overpowering
influence.
The fixed math looks like this:
b = b + w0 * (n0 - b) + w1 * (n1 - b) + ...
Fixes#5584.
fcbc6d43a1