Fix Deterministic blending with Dominant/Recessive doesn't have initial value even if there is no Discrete track

[TokageItLab]

Follow up #89389. Fixes #92127.

I listed the behaviors, some of which were problematic:

Deterministic	RESET Track	CallbackModeDiscrete	Is init value appeared in animation with lacking some tracks
true	None (but there is track in another animation)	Dominant	No, init value may be discrete
true	None (but there is track in another animation)	Recessive	No, init value may be discrete

Indeed, the Reset track can be Discrete, but if there is no Discrete track in the other animations, it should be considered Continuous. This causes problems in Deterministic blending, where the results may be inconsistent as #92127.

With this PR, the Dominant/Recessive mode will first check for the presence of a Reset track, and if there is none, it will check for the presence of blended Discrete track. So the list will be:

Deterministic	RESET Track	CallbackModeDiscrete	Is init value appeared in animation with lacking some tracks
true	None (but there is track in another animation)	Dominant	Yes, if "there is no other discrete track blended" and "continuous track is blended or blend amount is zero" / But not so, it is No since discrete value should be take precedence
true	None (but there is track in another animation)	Recessive	Yes, if "there is no other discrete track blended" and "continuous track is blended or blend amount is zero" / But not so, it is No since discrete value should be take precedence

[TokageItLab]

I was organizing the documentation on Discrete mode, and since the #89389 was invading the role of the Deterministic option and generating confusion, I have reorganized the behavior.

This causes #89163 to recur, but that is the intended behavior to ensure Deterministic option result consistency. So #89163 needs to be controlled by turning off the Deterministic option.

[TokageItLab]

Adding the is_init flag used by Dominant and Recessive instead of init_use_continuous to solve the issue overall.

I remember conversing with reduz a bit about this at the beginning of 4.0, but the problem with #92127 in the first place is that the reset animation does not play when the blend amount is 0.

The workaround is to either apply the reset animation even if the blend amount is 0, or to use a flag to apply the reset animation only once. This PR will use the former for Force Continuous and the latter for Dominant and Recessive. This will make the Dominant mode reset occur only once, so the problem in #89163 should not recur.

However, that init flag may not be very stable, since it changes depending on the timing of cache deletion in the absence of Continuous tracks. If real stability is needed, the timing of applying the Discrete value itself would need to be changed, but at this stage that change is too large.

[TokageItLab]

Now I make Recessive the default due to reevaluating the overall behavior.

In the past, we changed it once in #88492 because there was a problem with the Reset track taking precedence, but this should not be a problem now, and Recessive should be a better default because it does not have the flicker that can occur with Dominant, and it gives the same results as 3.x.

In there are two animations:

continuous_and_discrete.mp4

Force Continuous:

force_continuous.mp4

Dominant:

dominant.mp4

Recessive:

recessive.mp4

[fire]

We're not able to do a good review. We trust Tokage.

[TokageItLab]

I would like to include this in beta1 as this changes the default value, also consider the consistency with the blog post godotengine/godot-website#851.

[akien-mga]

Thanks!