Upgrade go-spatial/geom to latest

[jchamberlain]

This addresses the second point in #951. The main changes impacting this project are in the slippy package, and include:

• Using a grid for calculating extent, rather than relying on slippy.Tile directly.
• Removal of SRID-specific extent functions (Extent3857(), Extent4326()), thus requiring use a another tool for transformations.
• Buggy behavior in new slippy.RangeFamilyAt(), thus requiring keeping around the old version of that function.

[jchamberlain]

4 is not a valid y-value at zoom 2, and the geom upgrade makes this test fail unless a valid coordinate is used.

[ARolek]

@jchamberlain could you please split this PR into 2 commits: 1 for the vendoring of geom (and go.mod updates) and then a second one for the code changes? Makes review a bit easier.

[gdey]

LGTM

There are some minor code cleanups; that we can get to later. And we should make an issue around renaming slippy.PixelsToNative.

[ARolek]

@jchamberlain can you rebase against master? I just fixed the docker hub issue (and upgraded Go in the CI) so we can get the CI to pass. Then I can merge this in.

[coveralls]

Pull Request Test Coverage Report for Build 4ce028e55-PR-952

• 51 of 85 (60.0%) changed or added relevant lines in 8 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 46.855%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/tegola/cmd/cache/slippy.go	18	20	90.0%
cmd/tegola/cmd/cache/seed_purge.go	9	16	56.25%
server/handle_map_layer_zxy.go	12	24	50.0%
provider/provider.go	4	17	23.53%

Files with Coverage Reduction	New Missed Lines	%
provider/provider.go	1	54.29%
cmd/tegola/cmd/cache/cache.go	2	23.56%

Totals
Change from base Build 4d93140a4:	-0.05%
Covered Lines:	6571
Relevant Lines:	14024

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💛 - Coveralls