CCDIKSolver: Add blendFactor support.

[anishwij]

Description

Added ability to control IK influence through blend factors:

• Global blendFactor in update() method
• Per-chain blendFactor in updateOne() method
• Default value of 1.0 maintains backward compatibility

Use Case
I was developing an animation + procedural animation system and needed smooth interpolation between animation clips and IK implementation. Since the overrides happen in the CCDIKSolver, modifying it to support blending was necessary.

Usage Example

ikSolver.updateOne(indexFingerIk, indexBlend) // Control single IK chain
ikSolver.update(globalBlend) // Control all IK chains globally

CS_009266.2025-01.mp4

Alternative Approaches Considered

ikSolver.blendOne(ik, factor)
ikSolver[0].setBlendFactor() // target specific link
// or

const iks = [{
   target: 4,
   effector: 3,
   links: [{...}]
   iterations: 10,
   blendFactor: 1 // default blend factor
}]