Fixed interpolation for discrete types

[DavidSichma]

This fixes the animation player interpolating discrete types like Vector2i.

int: int is now interpolated using Math::round() instead of standard cast to int.
This aligns better with real types. Also only (-.5,0.5) is mapped to 0 instead of (-1,1) so interpolation is more 'linear' around zero.

Other types have their formula corrected. They use the same rounding behavior as int for consistency.

Somewhat unrelated: Should Variant::blend() be dropped? I can not find any uses in the engine, but it is exposed to GdNative. It does less than Variant::interpolate() and contains more bugs.

Edit: For testing be sure to interpolate values other than 0, otherwise the changes aren't visible.

[akien-mga]

I believe this might have been superseded by some PRs by @TokageItLab, could you confirm?

[TokageItLab]

I refactored it, but the algorithm was still retained, so I don't think it is a replacement for this PR.

At a quick look this change seems to make sense. But we need to test how ilerp is done elsewhere and whether this will cause problems (well, still I feel positive towards this change).

[TokageItLab]

cc @KoBeWi I think this relates to also tween.

[KoBeWi]

Tweens have a problem (which is not a bug) with interpolating sprite's frames (although it's the same with AnimationPlayer). When you interpolate from 0 to last_frame - 1, the last animation frame will be displayed for one render frame before the interpolation ends (which is not ideal when you want to e.g. free the node after animation).

This short code showcases the issue:

var tween = get_editor_interface().get_edited_scene_root().create_tween().set_process_mode(Tween.TWEEN_PROCESS_PHYSICS)
tween.tween_method(func(v): print(v), 0, 10, 1)

The output is:

0
0
0
0
0
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
4
4
4
4
4
4
5
5
5
5
5
5
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6
6
6
6
7
7
7
7
7
7
8
8
8
8
8
8
9
9
9
9
9
9
10

Notice how 10 appears once.

I tested how this behaves with ilerp:

0
0
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
4
4
4
4
4
4
5
5
5
5
5
5
6
6
6
6
6
6
7
7
7
7
7
7
8
8
8
8
8
8
9
9
9
9
9
9
10
10
10
10

The first frame appears less times and the last frame appears more times.

While it's still not perfect, it improves the usability a bit I guess.
(by perfect I mean each number should appear same amount of times, but that's not mathematically correct how interpolation works, probably)

[TokageItLab]

Ah indeed. At least as far as we think about int, the current master's code may be more ideal for controlling frames in the timeline for AnimatedSprite, since the borders are clearer.

But for simple interpolation of values, a transition like this PR would be preferable. Well, allowing them to be switched would be ideal, but seems a bit too expensive. It is not impossible to do such a transition only for Vector2i, but it does not seem like a good idea from a consistency standpoint...

It might be better to leave them as they are for now.

[TokageItLab]

Superseded by #84815.