flesh out gradient shaders

Using these webpages as guides

  http://www.globelegislators.org/pdfjs/test/pdfs/alphatrans.pdf

  http://www.antennahouse.com/antenna1/wp-content/uploads/2015/07/background-image-gradient-1.pdf

flesh out the gradient shaders to include the PostScript Type 0
sampling function and to include radial gradients. This CL makes
rendering these pages agree with Adobe Reader output.

Some of these examples use an Extend array to clip the gradient.
Skia does not currently support this natively, so construct the
clip manually for now.

Other PDF pages may construct gradients using alternate mechanisms
-- this code will continue to be refactored as those come to light.

In particular, this CL sets up the clip and matrix differently for
axial gradients and radial gradients -- while it makes sense to do
it one way only, I prefer to defer until I have more examples to
work with.

R=dsinclair@chromium.org,tsepez@chromium.org

Review URL: https://codereview.chromium.org/1870463002

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