Add context to transform filters.

[zerebubuth]

Currently we pass the zoom, but it would be better to pass a whole context with useful information such as:

1. A logger object for information, warnings and non-fatal errors.
2. The bounding box of the query / tile.
3. The other coordinates (x, y) of the tile.

We should do this for the filter functions and the post-process functions.

[zerebubuth]

Refs tilezen/tilequeue#70.