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Only implemented for PNG.

Add a getter and setter, and sets the default to false in the
constructor. Also copies the setting in copyFieldsToOther.

Fix an indpendent bug where fDitherImage was not being copied in
copyFieldsToOther.

In SkScaledBitmapSampler::begin, consolidate the settings passed in
by passing a const reference to the decoder. The decoder can be
referenced for its settings of dither, unpremultiplied, and now
skipping writing zeroes. Update callers to use the new API. In png
decoder, rather than passing around a pointer to an initial
read of getDitherImage, and potentially changing it, look at the
field on the decoder itself, and modify it directly. This is a
change in behavior - now if that same decoder is used to decode
a different image, the dither setting has changed. I think this is
okay because A) the typical use case is to use a new decoder for
each decode, B) we do not make any promises that a decode does not
change the decoder a...