Make AnimationMixer consider Discrete for RESET track

[TokageItLab]

• Fixes Godot 4.3.dev4 bug - animation on AnimatedSprite2D not working if frame is present in RESET animation #89163

Deterministic sets the initial value if an animation are lacking some tracks from another animation, but this should not be done for Discrete tracks, so add some flagging when caching and initializing. It should behave as follows:

Deterministic	RESET Track	CallbackModeDiscrete	Is init value appeared in animation with lacking some tracks
true	Continuous	Dominant	Yes, init value is continuous clearly
true	Continuous	Recessive	Yes, init value is continuous clearly
true	Continuous	Force Continuous	Yes, init value is continuous clearly
true	Discrete	Dominant	No, init value is discrete clearly
true	Discrete	Recessive	No, init value is discrete clearly
true	Discrete	Force Continuous	No, init value is discrete clearly
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
true	None (but there is track in another animation)	Force Continuous	Yes, init value is discrete clearly by Force Continuous
false	Continuous	Dominant	No, Un-Deterministic doesn't process init value
false	Continuous	Recessive	No, Un-Deterministic doesn't process init value
false	Continuous	Force Continuous	No, Un-Deterministic doesn't process init value
false	Discrete	Dominant	No, Un-Deterministic doesn't process init value
false	Discrete	Recessive	No, Un-Deterministic doesn't process init value
false	Discrete	Force Continuous	No, Un-Deterministic doesn't process init value
false	None (but there is track in another animation)	Dominant	No, Un-Deterministic doesn't process init value
false	None (but there is track in another animation)	Recessive	No, Un-Deterministic doesn't process init value
false	None (but there is track in another animation)	Force Continuous	No, Un-Deterministic doesn't process init value

[lyuma]

I think I see the idea, that this code wishes to add an additional case to disallow continuous blending if there is no continuous RESET track but not in a force continuous blend.

When there is no RESET track for a particular value, I understand it seems to default to true, but I don't understand why it is forced to true when FORCE_CONTINUOUS is set

Anyway I'll mark it approved because I trust your judgement and I'm not sure anyone is familiar enough with the track cache to give a better review.

Basically, this is introducing additional state to the track cache, and there are more cases that set it to true than set it false, so I want to make sure it is correct.

[akien-mga]

Thanks!