Merge branch '34-use-map_d-to-determine-whether-fragment-should-be-discarded' into 'master'

Resolve "Use map_d to determine whether fragment should be discarded"

Closes #34

See merge request rsethc/slice!19
2 jobs for 14-support-normal-textures-aka-bump-maps in 1 minute and 11 seconds (queued for 1 second)
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passed #973742138
build-slice-builder-ubuntu

00:00:56

passed #973742139
build-slice-builder-windows

00:01:11