Layer: Always calculate precision as extent per width|height

[Algunenano]

Closes https://github.com/mapbox/mapnik-vector-tile/issues/289

• Changes how the resolution of the mapnik::query is calculated. Given the formula: X / width and X / height:
• !ds: X used to be 256. Now it's 4096.
• ds.type == datasource::Vector && vector_layer_extent declared: X used to be 256. Now it's equal to vector_layer_extent.
• ds.type == datasource::Vector && vector_layer_extent NOT declared: X used to be 256. Now it's 4096. ds.type != datasource::Vector && vector_layer_extent declared. Used to be 256. Stays the same. ds.type != datasource::Vector && vector_layer_extent NOT declared. Used to be 4096. Stays the same.

[flippmoke]

@Algunenano Can you update changelog with explanation of fix and who it will affect in this PR + bump major version. After this will merge and release.