Add ability to request Raw or Smooth Depth Type

[alcooper91]

Adds the ability to request raw or smooth depth type.

Note that further spec text refinements are proposed in the second commit, which we could opt to revert and only merge the first commit instead.

A few very minor editorial fixes are also included in the initial commit, based on errors indicated by https://api.csswg.org/bikeshed/

Addresses #51

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[alcooper91]

@toji / @bialpio I need to adapt this for the optionality we discussed at the F2F, but would you mind taking a look at the general structure of the proposed algorithm change? (Note that I haven't run this by other spec editors, so maybe I'm doing something completely wrong/broken).