Function fill 

pub fn fill(
    level: Level,
    path: impl IntoIterator<Item = PathEl>,
    affine: Affine,
    line_buf: &mut Vec<Line>,
    ctx: &mut FlattenCtx,
    cull_bbox: RectU16,
)

Flatten a filled Bézier path into line segments.

Open subpaths and culling

Open subpaths in the input path get closed by connecting the last endpoint in the subpath to the starting point. The output lines in line_buf describe the flattened path, but these lines may describe open subpaths, as some path elements may have been culled.

For example, consider the following, where the box describes the viewport, a path is marked by *, and the region to be filled in the viewport is shaded. For ease of drawing the ASCII art, the path elements are all lines (PathEl::LineTo), but the same also holds for Bézier path elements.

   ---> winding scan direction

                  * * * * *
                *         *
   ---------- * -----     *
   |        *░░░░░░░|     *
   |      *░░░░░░░░░|     *
   |    *░░░░░░░░░░░|     *
   |  *░░░░░░░░░░░░░|     *
   |*░░░░░░░░░░░░░░░|     *
  *|░░░░░░░░░░░░░░░░|     *
*  |░░░░░░░░░░░░░░░░|     *
*  |░░░░░░░░░░░░░░░░|     *
*  ------------------     *
*                         *
*                         *
*                         *
*                         *
*                         *
*                         *
*                         *
* * * * * * * * * * * * * *

Because the winding scan direction is from left to right, only the left-of-viewport and diagonal lines matter in later stages of rendering for the winding number and pixel coverage. The other three lines can be culled.

                  *
                *
   ---------- * -----
   |        *░░░░░░░|
   |      *░░░░░░░░░|
   |    *░░░░░░░░░░░|
   |  *░░░░░░░░░░░░░|
   |*░░░░░░░░░░░░░░░|
  *|░░░░░░░░░░░░░░░░|
*  |░░░░░░░░░░░░░░░░|
*  |░░░░░░░░░░░░░░░░|
*  ------------------
*
*
*
*
*
*
*

It is important to keep these flattened subpaths open after culling, as closing the subpaths might yield different geometry like the following.

                  *
                **
   ---------- * *----
   |        *░░*    |
   |      *░░░*     |
   |    *░░░░*      |
   |  *░░░░░*       |
   |*░░░░░░*        |
  *|░░░░░░*         |
*  |░░░░░*          |
*  |░░░░*           |
*  --- * ------------
*     *
*    *
*   *
*  *
* *
**
*