SkSplicer: implement load_tables and matrix_3x4

These are enough to splice interesting SkColorSpaceXform pipelines.

SkSplicer_stages.cpp is similar to but still intentionally distinct from
SkRasterPipeline_opts.  I hope to unify them next week.

unaligned_load() is nothing tricky... just a little refactor.

Change-Id: I05d0fc38dac985aa351d88776ecc14d2457f2124
Reviewed-on: https://skia-review.googlesource.com/7022
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
3 files changed