Feature request - native tile map support

[iliatimofeev]

While discussing on vega #1212 I found another way (#1324) to implement tile map layer using current syntax of vega 3. I suggest to include some support of tiles into vega-light. Example of implementation on bl.ocks vega3 tiles layer

[domoritz]

I was amazed when I saw the proposal. It's fantastic!

I think what makes the most sense it to add a tile transform as @jheer suggested and then adding support for tile layers in Vega-Lite. Do you agree?

[iliatimofeev]

I think you are right. Let's wait for special transformer it will simplify implementation.