Fix incorrect z coordinate on overzoomed tiles

[drewbo]

mapbox-gl-js version: 0.21.0

When calling querySourceFeatures past the max zoom of the source layer, the tiles returned are not in the viewport. They have the x and y values from the max zoom of the source layer but the z value from the actual zoom level. My recreation relies on some private layers but you can also see the error in the tile boundary calculation (11/328/794 is correct, 11/164/397 has the correct x/y for zoom 10):

I believe the reparseOverscaled ternary operator is reversed in the TilePyramid; when swapping z and actualZ locally the problem was resolved. I can add a PR but not sure of a good test case

[lucaswoj]

See #1912 for more context