mvt: removing tegola artifacts and adding docs

[ear7h]

I removed tegola.Tile from the function signature. The encoding function now takes an geom.Extent that is used to represent the extent of a tile. It also takes the size of the ouput tile in pixels.

This approach makes it so the encoder does not have to worry about the geom.Geometry's SRID. The passed in geom.Extent is expected to be in the same projection as the `geom.Geometry.

Todo: I need to add a comment in PrepareGeo, I'm not sure what happens when the geometry goes outside the tile. Are geometries allowed outside of the tile?

[ARolek]

@ear7h if a geometry goes outside a tile (i.e. a point outside the tile) it will be removed by the clipping process. If the geometry covers the tile, it will consist of 4 points, one at each tile corner.

[ear7h]

@ARolek okay, I'll add a note about clipping in the readme and inline docs.

[gdey]

One minor suggestion

[ARolek]

LGTM

[ear7h]

This is good to merge

[coveralls]

Pull Request Test Coverage Report for Build 244

• 7 of 14 (50.0%) changed or added relevant lines in 1 file are covered.
• 16 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 57.321%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
encoding/mvt/prepare.go	7	14	50.0%

Files with Coverage Reduction	New Missed Lines	%
encoding/mvt/prepare.go	16	21.43%

Totals
Change from base Build 239:	0.0%
Covered Lines:	5414
Relevant Lines:	9445

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💛 - Coveralls