Implement custom Leaflet layer for exporting bitmap

[yongjun21]

Custom Leaflet layer that extends Polygon layer by exposing a .getBitmap() method for exporting bitmap of drawn polygon.

When the layer draws a polygon to canvas, it stores a Path2D object which can be used to create an offscreen canvas which in turn can be used for exporting bitmap

It works by monkey patching the renderer's canvas context to spy on the path methods.

declare module CustomBitmapLayer {
  interface Bitmap {
    x: number;
    y: number;
    width: number;
    height: number;
    getImageBitmap: () => ImageBitmap;
    getImageData: () => ImageData;
    getImageUrl: () => Promise<string>;
  }

  export class CustomBitmapLayer extends L.Polygon {
    getBitmap(padding?: number): Bitmap | null;
  }

  export function createCustomBitmapLayer(): CustomBitmapLayer;
}