Mosaic & Tiles with Buffers result in transparent pixels

[lseelenbinder]

If a tile on the edge of a MosaicJSON "meta-tile" is requested with a buffer, the edge of the tile is empty, because assets_for_tile doesn't account for the buffer.

I'm happy to take a stab at fixing it by adding a buffer input to the method, but I wanted to make sure I wasn't missing something or running into any sharp edges.