Implement segmenting of Line2D for better fidelity when using width curves

[chryan]

Currently, Line2D doesn't render the correct curved geometry that would intuitively expectat from how define you width curve - it doesn't accommodate well for when the number of points on your line are less than the number of points on your curve.

Here's an example of the problem described:

And this is what I expect it to look like:

I have a pull request here that fixes it by generating additional points on a line based on the number of segments derived from the curve bake resolution: godotengine/godot#95541

Along with the problem described above, my pull request also adds an extra curve_offset property which allows you to do some neat Line2D animations with the curve width (see example videos below).

Segment produce better 'wavey' lines when more curve control points exist than line points:
https://github.com/user-attachments/assets/35a1fe1a-bd1f-4313-bb29-d7d7adb75b57

Curve offset adjusts the sampling position of the curve:
https://github.com/user-attachments/assets/d66792df-39c9-4945-93ab-47e96d49cfcd

You can also animate the curve offset to produce some neat line effects:
https://github.com/user-attachments/assets/e1e2d108-ad7d-4fdd-83b9-3629abf76da2

Other Notes:

• It's possible for the logic to move into the LineBuilder if it's deemed that that's the better place to implement this feature. I'm just not currently familiar with the math behind how that works so I opted for the simpler solution
• The number of segments corresponds with the curve bake resolution instead of a being separate property - I didn't see it being very useful to keep those values independent

[chryan]

Made a proposal here instead:
#10452