provider/postgis: replace Mapnik pixel_width/height and scale_denominator SQL tokens

[olt]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 1329

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 19 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.04%) to 44.056%

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Files with Coverage Reduction	New Missed Lines	%
provider/gpkg/binary_header.go	2	100.0%
cmd/tegola/cmd/cache.go	17	27.07%

Totals
Change from base Build 1327:	0.04%
Covered Lines:	5129
Relevant Lines:	11642

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💛 - Coveralls

[ARolek]

@olt I had a few minor changes which I made in PR #503, mainly making the tokens case insensitive so we could stay in alignment with the uppercase tokens already in tegola. Thanks for the PR.