Add jpeg encoder alpha handling option

This instructs us on how to encode jpegs when the src
image has alpha.  The original behavior is to ignore
the alpha channel.  This CL adds the option to blend
the pixels onto opaque black.

Note that kBlendOnBlack and kIgnore are identical
unless the input alpha type is kUnpremul.

Bug: 713862
Bug: skia:1501
Change-Id: I4891c70bb0ccd83f7974c359bd40a2143b5c49ac
Reviewed-on: https://skia-review.googlesource.com/15817
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
diff --git a/rainbow-gradient.png b/rainbow-gradient.png
new file mode 100644
index 0000000..4e18a32
--- /dev/null
+++ b/rainbow-gradient.png
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