SkPngCodec: Add support for 16-bit pngs (step 1)

Android plans to use 16-bit png to encode higher precision assets.

This CL should not change any behavior or cause diffs on Gold.
It simply moves the 16-bit -> 8-bit strip from libpng to SkSwizzler.

As a follow-up, I plan to add support for 16-bit input to
SkColorSpaceXform.  This will require a new swizzler function that
just samples or subsets 16-bit values (but does not strip to 8-bit).

An alternative implementation could avoid the additional swizzler
functions by deciding whether or not to call png_set_strip() at
decode time (we would still need the swizzler fn to sample/subset
16-bit values).  I find this strategy to be cleaner than that.
I would rather handle 16-bit rgb(a) all the time than *some* of
the time.  And this is implementation is also more efficient
than libpng.  Though it is also more skia code.

Gray and gray alpha are left alone until I know whether anyone
wants high precision gray support.

b/32984164

Change-Id: I44e307473526de3f4bba06879c5fffa25d480f56
Reviewed-on: https://skia-review.googlesource.com/6020
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>

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