Make SkCreateRasterPipelineBlitter() not fail.

Make SkCreateRasterPipelineBlitter() not fail.

Mostly this is about extending the fallback in SkShader::appendStages()
to cover more cases, and making sure subclasses call appendStages() so
they can get the fallback, not onAppendStages() directly.

We still need to watch for SkShader::makeContext() failing in the
fallback itself, so sadly SkShader::appendStages() may still fail.

Change-Id: I2314b234a24bdcecac401a385ce050d7fdf0a83e
Reviewed-on: https://skia-review.googlesource.com/16369
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>