[rshapes] Fix multisegment Bezier splines.

[spelufo]

It seems to me that these functions are wrong, if you step the index by 1 you move to a control point instead of the next segment.

[raysan5]

@spelufo Did you test it? Could you share some code example?

[spelufo]

I did, through raylib-go:

package main

import rl "github.com/gen2brain/raylib-go/raylib"

func main() {
  rl.InitWindow(800, 600, "raylib [rshapes] example - bezier")
  defer rl.CloseWindow()

  rl.SetTargetFPS(60)

  for !rl.WindowShouldClose() {
    rl.BeginDrawing()
    rl.ClearBackground(rl.RayWhite)
    points := []rl.Vector2{
      {0.0, 0.0}, {50.0, 0.0}, {50.0, 50.0},
      {100.0, 50.0}, {100.0, 100.0}, {250.0, 250.0},
      {500.0, 500.0},
    }

    // Does the wrong thing.
    rl.DrawSplineBezierCubic(points, 5.0, rl.Green)

    // Proposed implementation:
    for i := 0; i < len(points) - 3; i += 3 {
      color := rl.Orange
      p := points[i]
      rl.DrawSplineSegmentBezierCubic(p, points[i + 1], points[i + 2], points[i + 3], 5.0, color)
    }

    rl.EndDrawing()
  }
}

[raysan5]

@spelufo I'm trying this fix with example shapes_splines_drawing, replacing current DrawSplineSegmentBezierCubic() in a loop by the proposed solution but it doesn't seem to work as expected. Please could you review it?

Drawing code I'm using:

Vector2 fullPoints[14] = {
    points[0],
    control[0].end,
    control[1].start,
    points[1],
    control[1].end,
    control[2].start,
    points[2],
    control[2].end,
    control[3].start,
    points[3],
    control[3].end,
    control[4].start,
    points[4],
    control[4].end
};

DrawSplineBezierCubic(fullPoints, 14, splineThickness, RED);

The expected result is:

[spelufo]

I've fixed the example code. I tested that the previous version of DrawSplineBezierCubic didn't work right. The new version works with this example code. The sequence of points passed to it must be points[0], control[0].start, control[0].end, points[1], ..., points[pointCount-1], and the length of control must always be pointCount - 1.

[spelufo]

Regarding the "connections", the gaps left on the points between segments, one way to cover them up would be to have DrawSplineBezierCubic draw a circle at the points connecting segments of diameter equal to the thickness of the lines. Maybe not the most efficient way to do it but it would cover the kinks.

[spelufo]

I've gone ahead and did that. It is ready to merge if you want.

[spelufo]

@raysan5 ?