alternate approach to unpremul scalePixels()

We want to keep the clamps in SkImageShader, and keep unpremul
scalePixels() happy too.

So we extend SkImageShader's internal API to allow specifying an output
alpha type, which controls whether we premul or unpremul and how we
clamp.  scalePixels() uses this to draw instead of a drawBitmap() call.

Sort of backwards of usual, we opt our local builds into
SK_LEGACY_HIGH_QUALITY_SCALING_CLAMP (and Google3 is already defining
this).  Then to rebase Chromium we will _define_ this in Chromium's user
config, fold it through as if always defined in Skia, then finally
remove the definition from Chromium's user config.

Change-Id: I38035f0886f79700e7301c3c6042ce362c771d96
Reviewed-on: https://skia-review.googlesource.com/90480
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

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