This PR fixes two issues:

1. Before ST_AsMVTGeom(geometry, ST_TileEnvelope(zoom, x, y), extent, buffer, true) function read buffer from layer yaml definition buffer_size (e.g. 256 for layer place). Which is in pixels but should be in vector tile unit because it is compared to extent (4096). Related to PR Bug fix tile_to_bbox(): use pixel_width instead of extent. #333.

2. When replacing !pixel_width! and !pixel_height! in Mapnik query, self.pixel_width and self.pixel_height are used. That means that a constant value pixel_scale is used throughout all zooms. However I checked logs from Mapnik rendering and following values for pixel_width are used:

   Zoom	pixel_width
   1	78271.5
   2	39135.8
   3	19567.9
   4	9783.94
   5	4891.97
   6	2445.98
   7	1222.99
   8	611.496
   9	305.748
   10	152.874
   11	76.437
   12	38.2185
   13	19.1093
   14	9.55463

   I modified the tool so it calculates pixel_width (which is actually zoom resolution meters/pixel) for each zoom_level. Using constant value for pixel_width leads to having different number of features in a same tile rendered via PgQuery and Mapnik.