[Breaking][Anim Component] Layer blending update

[ellthompson]

This PR adds the option to toggle the normalisation of layer weights on or off.

If weight normalisation is turned on the algorithm functionality is the same as before:

• Weights of all layers sum up to 1 (normalised by total weight)
• Overwrite layers set all previous layer weights to 0
• The resulting animation is a linear combination of all layer values and their weights

If weight normalisation is turned off the following effects take place:

• Weight values are not normalised by their total weight
• Layer weights are clamped between the 0 and 1 ranges
• Additive layers add their values on top of previous layer values based on their weight value
• Overwrite layers replace a portion of previous layer values based on their weight value
• Overwrite layers no longer set the weights of previous layers to 0

The default setting going forward will be not to normalize weights. As the anim component will no longer use the previous blending algorithm as the default, this PR becomes a breaking change. If users wish to use the old blending algorithm, they will be required to set the normalizeWeights property to true.

This PR creates new public API:

/**
 * Blend from the current weight value to the provided weight value over a given amount of time.
 *
 * @param {number} weight - The new weight value to blend to.
 * @param {number} time - The duration of the blend in seconds.
 */
pc.AnimComponentLayer#blendToWeight

/**
 * If true the animation component will normalize the weights of its layers by their sum total.
 *
 * @type {boolean}
 */
pc.AnimComponent#normalizeWeights

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[mvaligursky]

The default setting going forward will be not to normalize weights. As the anim component will no longer use the previous blending algorithm as the default, this PR becomes a breaking change. If users wish to use the old blending algorithm, they will be required to set the normalizeWeights property to true.

I wonder if we could handle this as non-breaking change? If normalizeWeights is undefined (existing assets), we should assume it's true. For new assets this will be initialized to false.

[mvaligursky]

Looks good now, I'm approving it.

But before the merge, I would just want to double check this is a breaking change and cannot be done as non-breaking. I think you explained it cannot, but please consider it one more time.