Merging getScreenShot and takeSnapshot methods

[ggalambas]

This has the goal of reducing the AtlasController interface, as both methods would do the same for different Map Providers.

The previous parameters x and y are now the anchor parameter of the viewpoint. The previous width and height are now the size. Example:

// old
final screenshot = await atlasController.getScreenShot(
  x: 0,
  y: 1,
  width: 200,
  height: 300
);

// new
final screenshot = await atlasController.getScreenShot(
  context: context,
  size: Size(200, 300),
  viewpoint: Viewpoint(
    anchor: ScreenCoordinates(x: 0, y: 1),
  ),
);